Thanks for the bug report, unfortunately I have not been able to reproduce this issue so far. I tested under three different notebooks, which use either Nvidia or Intel chipsets, and with different positions for the primary and secondary monitors.
You say that the windows open on the correct monitor initially but then move to the other monitor. This does seem to indicate that mIRC is opening them in the correct location and that your graphics software is relocating the windows. Perhaps it is doing so because it assumes that most applications are not multi-monitor aware. This is just a guess though.
