[chrismeller]

It really comes down to exactly what you mean by "decentralized". In the case of IRC it is, in a sense, decentralized in that it need not be a single server operated by a single person/company/whatever. Some of the larger networks have dozens of servers hosted by multiple different entities and spread out across the globe. As someone else pointed out (and had their comment killed, for some reason) the concept of a "netsplit" exists specifically because of the decentralized nature of the protocol - there exist scenarios in which the server you're connected to can no longer communicate with some or all of the other servers on the network and so the network is split in two with users continuing to exist on either side but unable to communicate with each other.

Basic, plain, no-frills IRC is also decentralized in that there is not a single user registry (since there is no user registration mechanism at all). Most of the time you're going to add on services (Nickserv and Chanserv at a minimum) and that's going to involve a single point of failure and control for authentication, so there is a potential downside there, but it's still arguably better than it being controlled by a single for-profit company that interprets and enforces laws in different ways.

Whether or not those caveats are in any way relevant to the use case you care about really depends on what that use case is... I would hazard to say that for most people, no matter what the use case, it would be more than sufficient - piracy rings, cyber criminals, and millions of nerds the world over have used IRC to communicate on a daily basis for decades, so there's quite a track record.