beware ircd can in fact run on any system which can run windows - that is a slow 486 for windows 95. TCP/IP protocol must ofcourse be installed.
for a 24/7 server with a lot of users, i recommend a fast pentium and windows NT or 2000 server. on windows 2000 pro, more than about 1500 users on a single server is not recommended, but it may be completely different for 2000 server - i haven't tested on this. more users can be done by linking multiple servers.