mIRC is currently 32bit. It performs some 64bit calculations and make some calls that use 64bit values but these are through standard Windows APIs that are not 64bit specific.
As for clean versions of mIRC, that's tricky because I don't have copies of older releases. The only versions I can guarantee are the digitally signed versions from v7.1 onwards. Some websites such as oldversion.com have older versions of mIRC and virus scan them.
I haven't tried Raspberry Pi before - which model are you using?
