Make sure you read
this thread, it contains a lot of information.
Routers and firewalls are the most common problem. Default DCC ports for mIRC are 1024-5000, but you only need two preferably in that range, but not mandatory. Might be easier also if they are consecutive, for example, 4000-4001. When these are set in the router and/or firewall, they also need to be set in mIRC.
For mIRC 6.14 it's:
ALT+O > Connect > Options > [Advanced]For mIRC 6.12 and below it's:
ALT+O > DCC > OptionsThere are also many suggestions related to DCC issues at
mIRC.org and in the
mIRC FAQ.
G'luck with gettin' it sorted
Regards,