HMAC is defined in RFC 2104, and its fairly straight-forward to implement in mIRC. Infact, I used to use this snippet for challenge-auth on QuakeNet:
Code:
alias -l hmac_md5 {
  bunset &opad
  bunset &ipad

  bset &key 64 0
  bset -t &key 1 $2

  bcopy &ipad 1 &key 1 64
  bcopy &opad 1 &key 1 64

  var %i = 1
  while (%i <= 64) {
    bset &ipad %i $xor($bvar(&ipad, %i),$base(36,16,10))
    bset &opad %i $xor($bvar(&opad, %i),$base(5c,16,10)) 
    inc %i
  }
  bset -t &ipad $calc($bvar(&ipad,0) + 1) $1
  bset &opad $calc($bvar(&opad,0) + 1) $digest($md5(&ipad, 1))
  return $md5(&opad,1)
}

alias -l digest {
  var %ret = ""
  var %i = 1
  while (%i < $len($1)) {
    %ret = $+(%ret,$chr(32),$base($mid($1,%i,2),16,10))
    inc %i 2
  }
  return %ret
}


Replace $md5 with $sha1 and you should be fine; all you need to know now is what your HMAC-components are (pass them as $1 and $2)