Element. It uses the Matrix protocol, which can even bridge to IRC.

There are irc clients on iOS.

I’ve been using The Lounge as my IRC client since ~2015 (and even contributed a fair bit initially). It’s a self-hosted webapp solution that’s pretty easy to set up.

On Android, it’s basically seamless: I get push notifications and everything, and performance is entirely sufficient. Some people on my instance are using it daily on iOS and it works alright although Apple’s refusal to support WebPush and full PWAs have made it a little less convenient to use, but that’s supposed to change with the upcoming iOS version.

Apart from IRC, there’s decent Matrix clients as well available for iOS, and it’s where most of the IRC community is slowly migrating to when they outgrow IRC. Most IRC networks have Matrix bridges as well so you’re not losing on IRC.

Revolt.chat is pretty nice

Don’t use Apple myself, but can’t you access anything via a web app, even on a restrictive system like iOS?