The in-game MP3 player you mentioned, by the way, isn't actually an in-game MP3 player. It's an external program that looks at your log file and can execute its own functions based on what gets added when you type certain things.
I suppose you could write some kind of mIRC plug-in that would do something similar, but it would be send-only. Member(s) of the EQ2 development team have clearly stated that using any external program to insert data into the game is a violation of the EULA/TOS (paraphrased). This means there would be no way to see what other people are typing in IRC, making it effectively useless.
It may be possible to have a window running on top of the full-screen game with some in-depth DirectX programming; but even if it is, it would really have nothing to do with EQ2.
