mIRC is a popular Internet Relay Chat (IRC) client used for chatting and communicating with others on IRC networks. The version 6.35 you're referring to is an older version.
This made mIRC relatively easy to keygen. A key generator (keygen) would run a brute-force or reverse-engineered algorithm to produce an infinite number of seemingly valid keys. Popular cracking groups like Core, PARADOX, and TSRH released keygens specifically for mIRC 6.35. Mirc 6.35 Registration Code
The most reliable way to register any version of mIRC, including older ones, is through the Official mIRC Registration Page. Cost: A single-user license costs $19.95. mIRC is a popular Internet Relay Chat (IRC)
Suppose you want to relive the year 2006 safely. Here is how to do it without hunting for registration codes: A key generator (keygen) would run a brute-force
The "Moo" Mystery: Khaled Mardam-Bey, the creator, famously joked that the "m" in mIRC stood for "moo".
Alternatives to MIRC