If it lets you use command line parameters then you could catch those using mIRC like so:

Code:
on *:start:{ echo -a CMD LINE : $cmdline }


So you could tell it to run e.g. "C:\program files\mIRC\mirc.exe" batch_finished

Code:
on *:start:{
  if ($cmdline == batch_finished) { ; Do stuff with the batch file ... }
}


The only problem with this is it will run a new copy of mIRC each time rather than the currently running one.