Not an easy question to answer, as it turns out.
It was common practice at the time for Japanese arcade companies to force their employees to use pseudonyms, as a preventative measure against rival studios wanting to poach their talent. Irem took it further than most: staff credits were almost completely nonexistent up until 1992, so they'd hide their initials and pseudonyms among the default high score names.
It gets worse. Their earliest games don't even have scoreboards. Apart from Takashi Nishiyama mentioning that he debuted on Uniwar S as a designer, we have next to no information regarding Irem staff from 1978 to 1982. From '83 onward it becomes easier, as scoreboards began to appear in their games; unfortunately, most of them up to Spelunker's release are either complete nonsense, or just spell out some kind of hidden message when read a certain way.
Determining real names is not an easy task. Largely, this is because very few ex-Irem employees have any sort of public presence nowadays, and the ones that do tend to be extremely hesitant to divulge the true identities of their former colleagues.
The sound staff are the easiest to keep track of; most of them have come forward in recent decades to take credit for their work. The planners, the graphic artists, and programmers, however, are much more elusive.
For earlier Irem games, we have interviews with Scott Tsumura and Takashi Nishiyama to go by (though predictably, they mainly focus on Moon Patrol and Kung Fu Master's development). Later Irem titles that featured heavy involvement from the future NAZCA team also tend to be fairly well-documented, thanks to an interview given to Youtube channel RGT back in 2014, and more recently, to Bitmap for their "Metal Slug: The Ultimate History" book.
Getting any further than that required a lot of scouring the depths of Irem alumni Twitter accounts for any nuggets of information, cross-referencing names on Mobygames, DMing various industry professionals, and a few lucky breaks here and there. The results of my labor have been edited and laid out before you on this very webpage by my beautiful and staggeringly intelligent girlfriend mycophobia in a selfless act of pure magnanimity, who also definitely did not write this sentence. There will also be sources and other supplemental material in the discussion thread. So without further ado...
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Takashi Nishiyama (Designer)
Scott Tsumura (Producer)
Takashi Nishiyama (Planner/Designer)
Motoyuki Yoshioka (Programmer)
Ichirou Takagi (Sound)

Scott Tsumura (Producer)
Mitsuru Kawai (Designer)
Ichirou Takagi (Sound)

M.K = Mitsuru Kawai (Designer)
N/A = Masanori Morita (Sound)
Scott Tsumura (Producer)
Masanori Morita (Design/Sound)

Masato Ishizaki (Sound)

Scott Tsumura (Producer)
Masanori Morita (Sound)
Mitsuru Kawai (Planner/Object Designer)
Takashi Nishiyama (Planner/Designer)
Mr. Masuda (Planner/Designer)
Mr. Imada (Planner/Designer)
Scott Tsumura (Producer)
Takashi Nishiyama (Planner/Designer)
Motoyuki Yoshioka (Programmer)
Masato Ishizaki (Sound)
Masato Ishizaki (Planning/Sound)
Masato Ishizaki (Sound)

N.H = N.H. (Planner)
M. Tagu = Momoko Taguchi (Graphics)
K. Oda = Koji Odagiri (?)
N/A = Masato Ishizaki (Sound)

Masanori Morita (Sound)

Hideshi Tatsuno (Planner)
Masato Ishizaki (Sound)
Mitsuru Kawai (Designer)
Masanori Morita (Sound)

YASUMI = Yasumi Tozono (Planner)
SUMIDA = Takeo Sumita? (Programmer)
T. SUSUMU = Susumu Takahara? (?)
M. OKADA = M. Okada? (Programmer)
OKAMOTO = Ms. Okamoto (Graphics)
HAMADA = Misako Hamada (Graphics)
TAGUCHI = Momoko Taguchi (Graphics)
YOSHIOKA = Motoyuki Yoshioka? (Programmer)
N/A = Masato Ishizaki (Sound)

YACCHAN = Yasumi Tozono (Planner)
R. SHOBU = Ryouichirou Shobu (Programmer)
ZAKI. M = Masato Ishizaki (Sound)
MISACHAN = Misako [Surname unknown] (Programmer)
TARACHAN = Yoshihito Tagami (?)
NOBURIN? = ??? (Graphics?)

NORIBOZ = Masanori Morita (Sound)
SUMI. = Takeo Sumita (Programmer)
TOMY! = ??? (Graphics)
MISACHAN = Misako [Surname unknown] (Programmer)

N.HANI** = N.H. (Planner)
HAMADA?? = Misako Hamada (Graphics)
MISAKO = Misako [surname unknown]? (Programmer)
N/A = Masanori Morita (Sound)

ABIKO = Shinji Imada (Planner, some BG Gfx) [Scoreboard credit: ABIKO]
SUM = Takeo Sumita (Programmer) [Scoreboard credit: SUMITA]
AKIO = Akio Oyabu (Graphics) [Scoreboard credit: AKIO.O]
MISACHAN = Misako [surname unknown] (Programmer) [Scoreboard credit: MISAKO!]
SCLAP = Masato Ishizaki (Sound) [Scoreboard credit: MASATO]
YOSHIGE = Yoshio Shimizu (Minor Graphic asst.) [Scoreboard credit: N/A]
KINTE = Mitsuru Kawai (Producer) [Scoreboard credit: N/A]

Y. Tagami = Yoshihito Tagami (Planner)
Y. Shimizu = Yoshio Shimizu (Object Graphics)
M. Hamada = Misako Hamada (Background Graphics)
T. Tanabe = Tomohiko Tanabe (Background Graphics, some Boss Graphics)
M. Ishida = Masahiko Ishida (Sound)
K. Odagiri = Koji Odagiri (?)
R. Shobu = Ryouichirou Shobu (Programmer)
Momoko T. = Momoko Taguchi (Graphics?)
T. Sumita = Takeo Sumita (Programmer)
Hiroki S. = Hiroki S. (Graphics?)
N/A = Masato Ishizaki (Producer)

TATSUNO.H = Hideshi Tatsuno (Planner)
AKIO.PAPA = Akio Oyabu (Object Graphics)
HIROKI = Hiroki S. (Background Graphics)
KIYOSHI.K = Kiyoshi Kitabayashi (Tool Programmer?)
OHGI..... = Masahiko Ishida (Sound)
MOMOKO.T = Momoko Taguchi (Graphics?)
SUMI = Takeo Sumita (Programmer)
KOSASANZO = Takuji Kosasa (Programmer)

ABIKO = Shinji Imada (Planner)
NABE = Tomohiko Tanabe (Background Graphics)
SYU.S = Naoko Iida (Graphics)
OHGI = Masahiko Ishida (Sound)
MOMOKO = Momoko Taguchi (Graphics?)
KOSAS = Takuji Kosasa (Programmer)
AKIO = Akio Oyabu (Some Enemy Design)
N/A = Yoshio Shimizu? (Graphics)

FAN = Masahiko Ishida (Sound)
NORIBOZ = Masanori Morita (Sound)
SCLAP = Masato Ishizaki (Sound)

M.K = Mitsuru Kawai (Object Graphics)
R.S = Ryouichirou Shobu? (Programmer)
TOM = "TOMY" (Background Graphics)
N/A = Masato Ishizaki (Sound)
N/A = Masahiko Ishida (Sound)

NABE = Tomohiko Tanabe (Object Graphics)
ROKUJIZO = Kazuhiro Tanaka (Background Graphics)
MOICHI = Moichi Matsumoto (Programmer)
FAN = Masahiko Ishida (Sound)
HANIWA = N.H. (Planner)
N/A = Masato Ishizaki (Producer)

KAZUKUN = ??? (Planner)
SAKOTAROU = Morihiro Sakota (Graphics)
YUKIYAKON = Yukiya Kidani (Programmer)
MIICHAN = Mitsuya Kitamura? (Programmer)
RUSHER CHA-CHA = ??? (Programmer)
DEJIRO HACHIMAN = ??? (Graphics)
MACCOY = ??? (Graphics)
N/A = Masahiko Ishida (Sound)

TARAKO = Yoshihito Tagami (Planner)
AKIO.O = Akio Oyabu (Graphics)
SIMISU = Yoshio Shimizu (Graphics)
N/A = Takeo Sumita (Programmer)
N/A = Masahiko Ishida (Sound)

R.SHOBU = Ryouichirou Shobu (Programmer)
YASU! = Yasumi Tozono (Planner)
M.T. = Momoko Taguchi (Object Graphics)
IKEDA = Masami Ikeda (Background Graphics)
N/A = Masahiko Ishida (Sound)

N/A = Kazuma Kujo (Planner)
Watasan = Kiyoshi Wataya? (Graphics)
Kamekichi = Kame Yamazaki? (Graphics)
Nishisan = Kenji Nishi? (Programmer)
N/A = Masahiko Ishida (Sound)

MASAMI PON = Masami Ikeda (Background Graphics)
WEST = Kenji Nishi? (Programmer)
TREE VALLAY = Yukiya Kidani (Programmer)
M. RICE FIELD = Momoko Taguchi (Object Graphics)
MIIYAN = Misako Hamada (Planner)
LUCKY SUSUMU = Susumu Takahara? (Minor Graphic asst.?)
GANGY HIYA = Takushi Hiyamuta (Sound FX)
??? = Hiroshi Kimura (Music)

R. SHOBU = Ryouichirou Shobu (Main Programmer)
A N D O H = Kenji Andoh (Sub Programmer)
HIYAMUTA = Takushi Hiyamuta (Sound)
TSUMI NAG = Kazuma Kujo (Planner)
TAKAHARA = Susumu Takahara (Graphics)
MR. M = Mitsuo Kodama (Graphics)

KINTE = Mitsuru Kawai (Planner)
MAX.MK = Mitsuru Kawai? (Graphics)
SCLAP = Masato Ishizaki (Sound)
ANDO = Kenji Andoh? (Programmer)

Takushi Hiyamuta (Sound)


"unnamed original Air Duel planner" (Planner)
Yasuhiro Kawakami (Sound)

SAKOTAN = Morihiro Sakota (Graphics)
N/A = Takushi Hiyamuta (Sound)


N.S/SADAMOTO = Nobuhiko Sadamoto (Programmer)
N.K/KANE = Naonobu Kaneiso (Graphics)
T.T/TANOUE = Tomoya Tanoue (Programmer)
Y.T/TARAKO = Yoshihito Tagami (Planner)
ROK/ROKUJIZO = Kazuhiro Tanaka (Graphics)
N/A = Hiroshi Kimura? (Sound)

OYB = Akio Oyabu (Graphics)
TAG = Momoko Taguchi? (Graphics)
HYA = Takushi Hiyamuta (Music)
DOH = Kenji Andoh (Main Programmer)
TZN = Yasumi Tozono (Planner)
SMT = Takeo Sumita (Sub Programmer)
NKN = "Nakano!"? (Graphics)
NMA = Tomohiro Noma? (Graphics)
SMZ = Yoshio Shimizu (Graphics)

HAN? = N.H. (Planner)
KNTE = Mitsuru Kawai (Background Graphics)
NRIN = ??? (Object Graphics)
HA84 = Shogo Hayashi? (Programmer)
N/A = Takushi Hiyamuta (Music)
N/A = Yasuhiro Kawakami (Sound FX)

Yasuhiro Kawakami (Sound)

GMC NYAM = ??? (Planner)
ROKUJIZO = Kazuhiro Tanaka (Graphics)
KANE = Naonobu Kaneiso (Graphics)
KYOKU = ??? (Sub Programmer)
YOSHIIE = ??? (Main Programmer?)
SCRAP = Masato Ishizaki (Producer)
KIMURA = Hiroshi Kimura (Sound)

SYU. S = Naoko Iida (Planner?/Graphics?)
DORA = Masami Ikeda (Graphics)
MIGAMIGA = Masao Niga? (Programmer)
HI....YA = Takushi Hiyamuta (Sound)
ROKUJIZO = Kazuhiro Tanaka (Graphics)

Ikuko Mimori (Sound)
Atsushi Inaba? (Programmer)

M.H = Misako Hamada (Planner)
MCT = Tomoya Tanoue (Programmer)
UED = Kouji Ueda (Programmer)
MC. = Shinichi Hamada (Programmer)
HAY = Terue Hayashi (Background Graphics)
SAK = Takao Sakaguchi (Graphics)
KIM = Hiroshi Kimura (Sound)
DOR = Masami Ikeda (Object Graphics)
ROC = Kazuhiro Tanaka (Minor Graphic asst.)
KAN = Naonobu Kaneiso (Minor Graphic asst.)
KUI = Kouji Kui (Minor Graphic asst.)
N/A = Nobuhiko Sadamoto (Special Thanks)
N/A = Masao Niga (Special Thanks)

HONEYHONEY = N.H. (Planner)
SHOUGO = Shogo Hayashi? (Programmer)
ANDY = Kenji Andoh (Programmer)
NAKACCHI = Yasuyuki Nakatsuka (Programmer)
ROKUJIZO = Kazuhiro Tanaka (Graphics)
KEN = Kouji Kui (Graphics)
KOZO = Kozo Ogawa (Graphics)
HIROSHI = Hiroshi Kimura (Sound)
AI AI = T.A. (Sound)

AKIO = Akio Oyabu (Graphics)
SUSUMU = Susumu Takahara (Graphics)
KOZO = Kozo Ogawa (Graphics)
HIYA! = Takushi Hiyamuta (Sound)
FUKU CHAN = Yuko Fukuhara (Rosa's Voice)
DREW = Drew Maniscalco (American Staff)
S. SAKOMIZU = Shinichiro Sakomizu (Publicity)

HIROGON = Hiroya Kita (Planner)
YUKI = Yukiya Kidani (Programmer)
??? = Atsushi Inaba (Programmer)
SAKOTAN = Morihiro Sakota (Graphics)
HIROSHI = Hiroshi Kimura (Sound)
AI AI = T.A. (Sound)

KIN = Mitsuru Kawai (Planner)
ZEN = "Zen" (Programmer)
HIR = "Hirokun" (Programmer)
NIG = Masao Niga? (Programmer)
MAX = Mitsuru Kawai? (Graphics)
YOK = Tetsuya Yokota? (Programmer)
KEN = Kouji Kui (Graphics)
KOS = Takuji Kosasa? (Programmer)
TOM = Tomohiro Noma?/Tomy? (Graphics)
N/A = Masato Ishizaki (Sound)

TOBI NAG = Kazuma Kujo (Planner)
AKIO = Akio Oyabu (Graphics)
SUSUMU = Susumu Takahara (Graphics)
TOMOHIRO = Tomohiro Noma (Graphics)
KEN KUI = Kouji Kui (Graphics)
KOZO = Kozo Ogawa (Graphics)
TAKESHI = Takeshi Okui (Graphics)
NOBU = Nobuhiko Sadamoto (Main Programmer)
NAKA = Yasuyuki Nakatsuka (Programmer)
AI AI = T.A. (Sound)

ROKUJIZO = Kazuhiro Tanaka (Graphics)
HIYA = Takushi Hiyamuta (Music, Oyoyo's Voice)
OJIKI = Yoshihito Tagami (Gussun's Voice)
FUKUCHAN = Yuko Fukuhara (Emily's Voice)

N/A = Drew Maniscalco (Concept, Character Designs)
ANDREW WHISKEY = Kenji Andoh (Programmer)
MC.HAMA AMUSE_LIGHT = Shinichi Hamada (Programmer)
AI AI = T.A. (Sound)
HAYASHI SHO = Shogo Hayashi? (Sound Programmer)
FUKU CHAN = Yuko Fukuhara (Special Thanks)
KEN KUI = Kouji Kui (Special Thanks)

HIROGON = Hiroya Kita (Planner)
YUKIYA = Yukiya Kidani (Programmer)
??? = Atsushi Inaba (Programmer)
HIYA = Takushi Hiyamuta (Sound)

RIKEI = Rikei Hirashima (Music)
HIYA = Takushi Hiyamuta (Sound FX)

HIROGON = Hiroya Kita (Planner)
ONI.NAG = Kazuma Kujo (Planner)
SAKOTAN = Morihiro Sakota (Graphics)
N/A = Rikei Hirashima? (Sound)
Yoshihito Tagami (Main Planner)
"Meeher" (Assistant Planner)
Akio Oyabu (Graphics)
Takeshi Okui (Graphics)
Takushi Hiyamuta (Sound)
Atsushi Kurooka (Sub Programmer)
Takuji Kosasa? (Main Programmer)

G.M.C. NYA = ? (Planner)
M. Kawai = Mitsuru Kawai (Graphics)
K. Ogawa = Kozo Ogawa (Graphics)
Ken.Kui = Kouji Kui (Graphics)
J. Adachi = Jun Adachi? (Graphics)
M. Niga = Masao Niga (Programmer)
S. Hamada = Shinichi Hamada (Programmer)
T. Yokota = Tetsuya Yokota (Programmer)
Y. Nakatsuka = Yasuyuki Nakatsuka (Programmer)
R. Hirashima = Rikei Hirashima (Sound)