Guess it depends on what you mean with "accessible". In terms of accessibility for impaired users, IRC is a open protocol vs a service that disallows 3rd party clients. I'm sure there exists better options for IRC than Discord, and at the very least, IRC allows you to access it however you want, be it visual, textual, voice control or whatever.

Besides, Discord being a US company, need to prevent users from Cuba, Iran, Syria (and NK) and is also banned in a bunch of countries like China, UAE and Egypt. So in that sense, Discord seems less accessible than IRC.

Only remaining part is that Discord has a somewhat easier "getting setup" UX than IRC for younger users, as it's more similar to the type of services they're probably already use.