You will need identd enabled in mIRC and you need any firewall or router to allow (forward) port 113. If you haven't changed your mIRC settings, then the most likely scenario is that your router or firewall is blocking port 113. For help forwarding ports, visit
http://www.portforward.com/ and select your model of router or firewall. If the one you use isn't available, you should be able to get a general idea of how to do it by looking at another one as the method is going to be similar regardless of router or firewall.