mirc.exe is the process name for mIRC a shareware Internet Relay Chat client which seems to be the most popular IRC client. More information can be found here.

Quote:
mIRC is a shareware IRC client for Windows. It is developed and copyrighted by Khaled Mardam-Bey. mIRC is a highly configurable IRC client with all the goodies other clients on UNIX, Macintosh and even on windows offer, combined with a *nice* and clean user interface. mIRC offers full color text lines, DCC File Send and Get capabilities, programmable aliases, a remote commands and events handler, place sensitive popup menu's, a great Switchbar, World Wide Web and sound support, and... a lot more. mIRC is shareware but not crippled in any way...