sunslayer, see http://www.mirc.com/faq7.html under 'The ; and REM flag'
it's treated as a comment if mIRC recognizes /REM as the command before any code is evaluated, so you can even use //REM to mimic C style single line comments. you can even use this:
alias myalias {
echo -a line1
// this is a comment
echo -a line2
}
mIRC sees this as an empty command and ignores it without evaluating the rest of the line.