[9YearLurker]

So you have to go through passport controls to travel from, say, Italy to Ireland, even though they are both in the EU?

And thanks.

[Winniesboy]

Yes. A lot of people assume that the entire EU is open border. In fact it's only the Schengen countries - which, to add to the confusion, include 2 NON-EU states, Norway and Switzerland. As well as Ireland, the UK also was always outside Schengen before Brexit, as are EU members Romania, Croatia and Bulgaria. (There's a useful map clarifying this at post 198 to this aricle).

But on your original point - yes, having entered the Republic of Ireland, anybody can then freely cross the Ireland/Northern Ireland border, since this is open thanks to an entirely separate (ie non-EU) bilateral agreement between the UK and Ireland - the Good Friday Agreement.

However, one of the (many) outstanding questions still to be resolved in implementing Brexit is the nature of the customs (and possibly other) checks between Northern Ireland and the rest of the UK. There will undoubtedly be some, but the detail is still to be negotiated.