Irccloud is decent for consistent connectivity, although for android revolution irc is showing promise.

The IRC bridge of Matrix works pretty well.

Weechat is another popular choice, also offers access on mobile via Glowing Bear https://www.glowing-bear.org/

I ise irccloud, pretty amazing for few bucks

"IRC is by designed centralized"

Never encountered a netsplit, I see.

This is a misconception, normally IRC servers operate in a linked network sharing a pool of users and channels. Also there is no such thing as authentication in IRC, at least on a protocol level.

Not really, the ircnet servers (many of them) are linked together and they provide the service like that, you can connect to any of them.

An IRC channel is hosted by a network of federated servers (such as Freenode or EFNet). This is why netsplits happen when communication is disrupted between servers.

Some networks have bolted-on authentication to prevent people from riding netsplits and commandeering channels, but that's not a part of IRC itself.

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.

IRC is both centralized and decentralized.

Users join a single node (centralized) but that node is part of the wider IRC network (decentralized).