You could also send a specific string to a channel, which caused mIRC to log it to a file. Then a hyper-sensitive anti-virus would see it and quarantine mirc.exe :D
Probably EICAR. AV engines are only supposed to fire on EICAR when the file contains only magic string, but many are/were trigger happy and will alert if it appears anywhere.
https://en.wikipedia.org/wiki/EICAR_test_file