I do believe mIRC has problems running on a 64bit machine, other people have had problems with it.
There have been a number of threads which have suggested a 64-bit version of mIRC. The general opinion that I can see is that it's not deemed necessary at the moment due to the fact that it doesn't have general public use. (Unlike 16-bit -> 32-bit. That was necessary due to the fact that 99% of people have a 32-bit Windows OS now. 95, 98, ME, XP etc have it).
You can give your opinion about this in
this thread or
this thread (not both).
Have fun!
Regards,