Tested and works, but only for the main mIRC directory, not any sub-directories. If you want those as well, I can figure them out.
If this is going in your aliases, then as is will work. If in remotes, change the first line to alias backup {

Code:
 backup {
  if !$isdir(L:\tempbackup) {
    mkdir L:\tempbackup
  }
  if !$isdir($+(L:\tempbackup\,$date(ddmmyyyy))) {
    mkdir $+(L:\tempbackup\,$date(ddmmyyyy))
  }
  copy $nopath($mircdir*.*) $+(L:\tempbackup\,$date(ddmmyyyy),\)
}