Ideally, a program needs to be written to support being ran as a service - mIRC is not.
There are programs that will help you though. In this thread
it suggests using FireDaemon
, although this is only shareware.
There is also a utility in the Windows 2003 Server Resource Kit called 'Srvany', that might work for you. You can obtain the resource kit here
Be warned, mIRC may well not run well when run as a service.