AfaIk, the freedom of travel between the Republic of Ireland and Northern Ireland is part of the peace deal. So whatever protocol is in power now, needs to ensure this. Thus, as UK is no longer part of EU, in principle, there should be border controls between Northern Ireland and Britain.
They weren’t in Schengen before Brexit either, they had the “Common Travel Area” with the UK. Not sure what changed for after.
AfaIk, the freedom of travel between the Republic of Ireland and Northern Ireland is part of the peace deal. So whatever protocol is in power now, needs to ensure this. Thus, as UK is no longer part of EU, in principle, there should be border controls between Northern Ireland and Britain.