Of course there aren't passport checks on planes or boats between Great Britain and the Republic of Ireland, but that only applies to Irish and British citizens.

On planes/boats between Northern Ireland and Great Britain there aren't passport checks, but they can and do check your passport.

> Of course there aren't passport checks on planes or boats between Great Britain and the Republic of Ireland, but that only applies to Irish and British citizens.

And the only way to enforce that is to not enforce it all, or to check everyone for documents. Both cork and Dublin airports have a manned border check for all inbound flights (can’t speak for other airports) but a passport isn’t required. If you fly between European countries there’s no border check _at all_. (Your airline may check but that’s different).

> On planes/boats between Northern Ireland and Great Britain there aren't passport checks, but they can and do check your passport.

That’s a domestic flight so yeah. Anecdotally I don’t think I’ve ever been stopped for a passport check in 15 years of flying between the Uk and Belfast.