mIRC is free, it just bugs you when you start it up once you go over your trial period.
mIRC is not free and it even says so.
You can however continue using the trial for quite a while
Heres some from their FAQ:
Registration - mIRC is a shareware program. This system of distributing programs, as shareware, shows the true spirit of the Internet. You can download mIRC for free and give it a try. If during or after the evalution period you decide to continue to use mIRC, you're asked to pay a small registration fee. This will allow Khaled, mIRC's author, to go on developing and supporting mIRC. Once you registered one version of mIRC, you're allowed to use all future versions for free!WWW Link