Well, you can find most meanings if you look them up, though i'll tell you those for now!
IIRC is If I Recall Correctly. IRC is, indeed, the chat. Dakka, ehh, i never really thought out a definition, i guess it'd be firepower. More dakka!
Apparently it comes from the sound automatic guns make. Y'know, dakkadakkadakka. It comes from the orks in Warhammer 40k, and i just realize i should have remembered that!