I don't see this being spammy. I have no idea why everyone keeps saying this. I only imagine few people being on it. It would be really useful for me, because I could see chat without being on console and if people needed help, I could easily know when to get on.
That's the point. Only a few people will be on it at the start, but eventually more people will join and the chat from IRC will grow exponentially.
I use the term 'spam' as - I guess - chat flooding. When the chat just fills up your screen instantly. When there's 40 people in a server that happens anyways, but with a public IRC linked in it would be worse.
Then don't make it public. Have it a whitelisted, login only IRC network. Users can add themselves using /ircname <name> and then chat on irc using only that name. Inputting a new /ircname would remove the old one, and if having it login only is unwanted, then have /tempirc which times out after [a. the user leaves the server and on irc leaves the network or b. a certain amount of time elapses]. Simple.
I'd love to see an IRC mod/channel. It will help make the server a bit more lively when few people are playing, and if lots of people are playing (i.e. after map reset) then nobody will be on IRC anyway because they're playing. I don't think the channel will be flooded much. The idea of whitelisting yourself for IRC using an ingame command seems like the easiest way to not let outsiders in. Requires no attention from mods/admins and is very effective. Maybe we could also use a bot that automatically un-whitelists people who have not been online for a month or something to keep the whitelist clean. People banned from the server will also be banned from IRC so they don't spam and/or beg. Also, how about adding an optional client-side mod like TabbyChat? It makes the chat very customizable - you can resize it, format the text, add timestamps and, as the name says, use tabs. It can sort chat messages with regular expressions, so if we simply give ingame IRC messages a special colour or a prefix then it's easily possible to make a chat tab for IRC, and the normal chat will not be flooded by IRC messages. (Even without IRC I'd love to have this mod because when 40+ people are online the chat is already tetris level 50, and it can send multi-line messages <3 )
I agree with your first paragraph, and TabbyChat is considered, we need to test it with the chat plugin though.