Twitter hashtags has obvious similarities and differences to IRC channels.

However I have a strong feeling that I modified/used hashtags (as tagging, not as a channel) back in 2005/6.

I think I got the idea from another system (probably some early wiki) but I'm unable to recall now.

I thought it was to make keyword searches work on twitter

Sure but that notation looks obviously inspired by IRC to me.

when i started using twitter back in 2008, that's how it felt to me -- that # tags were basically irc rooms and twitter itself was just a big async irc