A handful of you might already know this trick, but I'm posting for those who don't know, or who are new to mIRC.
This trick will work whether your trial has expired or not. To reset your mIRC trial period back to 30 days, do the following:
Open the Start menu, execute "Run".
Type "Regedit" in the field. Hit OK.
Expand the "HKEY_CURRENT_USER" folder.
Expand the "Software" folder.
Look for a folder named "mIRC". Expand it.
Now, go ahead and delete the folder named "LastRun".
PROFIT! Your mIRC trial period is now back at 30-days.