There are a few flash options in mIRC already. Go to mIRC Options:
ALT+O > IRC > Options
and if you want mIRC to flash when you get a private message or channel message, check the appropriate boxes. I do not believe this will flash when someone joins the channel, but you could easily set this up with a simple script, one example being something like;
on *:join:#:{ flash JOIN }
THe /flash command makes mIRC flash in the taskbar, but only when mIRC is NOT the active application. See /help /flash for further information/options.
Hope this helps :-)
Regards,