From Khaled's FAQ
Question: In which language is mIRC written?
Answer: mIRC is written in a mixture of C/C++, in pure win32 code, under Borland C/C++ v5.02. I also compile and run the same code under Visual C++ 6.01 for testing purposes.
I used Borland C/C++ v5.02 because it allowed parallel development of 16/32bit code from the same code-base. This isn't important anymore because the 16bit mIRC is no longer being developed.
Question: How did you write the scripting language?
Answer: I had no experience with writing compilers or parsers when I first started working on mIRC. The scripting language grew organically, it wasn't designed. Users provided much input and over time the language became relatively organized and stable, though it still has some quirks. I would say that the language resembles C more than anything else.