By the way, using dll's doesn't mean you're limited to C/C++. You can use C#, J#, Python, Ruby, Perl, PHP, Java, VB.. pick your language. You may need a small C/C++ wrapper in some of those cases, since not all of those can produce dll binary files, but it's still doable. You could also just use COM, which all of those can interface with very easily. At that point, though, you would probably be wondering why you'd need to interface with mIRC at all, since the solution has nothing to do with the client in this case-- and you'd simplify your life by just doing it in C#/Ruby/Java from the start-- which is what I would have done.
As for C++ being slower.. I can't imagine that, since the only really convoluted programming in C++ that mIRC could otherwise do is socket stuff; most of everything else is pretty straightforward, especially when you're talking about looping over file data, reading in strings, etc.. mIRC literally copies the C api for filehandling (/fopen, $fread), so it really is no different.
