We've been running an IRC gateway from Matrix to Freenode almost since day 1 - originally it was a simple perl bot written by tm604 using Net::Async::Matrix; later it was rewritten by LeoNerd to be a bit more modular, and nowadays it's a full IRC/Matrix Application Service written in Node.js by Kegan.
Up until the end of last week the bridge was limited to synchronising a fixed number of channels between Freenode and Matrix (#matrix, #matrix-dev, #openwebrtc, #vuc and #hypothes.is, to be precise), as well as any PMs. But as of Friday, with huge thanks to the admins over at Freenode, we can now bridge any channel in Freenode through to Matrix.
Doing this is trivial - you just /join #freenode#channelname:matrix.org from a typical Matrix client - e.g. /join #freenode#Node.js:matrix.org will give you access to all of #Node.js via Matrix, effectively using Matrix as a great big distributed IRC bouncer in the sky ;)
There are a few limitations in the current setup:
!nick irc.freenode.net MrFlibbleto @appservice-irc:matrix.org. (This isn't actually turned on right now, but should be fixed shortly. See https://github.com/matrix-org/matrix-appservice-irc/blob/master/HOWTO.md#changing-nicks for details.
We'll keep chipping away at making the IRC<->Matrix mapping perfect, but in its current state it's still really usable. It's obviously beta still, but please give it a go and let us know in #matrix:matrix.org how you get on with it!