Never noticed a memory leak in AmIRC... I would use it for between six to 12 hours at a time. In fact, for a while I was leaving it on for days at a time. Could it be one of the MUI classes leaking?
The last time I used it regularly was probably a good three years ago. Saw heavy usage bewteen 1997-ish and 2004, with Kuang11 installed. I would assume several of the MUI custom classes used by AmIRC have seen updates if not out-right replacements since then.
It still runs very well for me when I fire it up, but it seems that the IRC social channels have all but been abandoned.