As described to me by an Irish immigration officer at Dublin Airport many years ago (well being a Dub what he actually said was "we cover der aarses, an' dey cover ouwers"). I was with a good friend who was from Malaysia and we were on holiday in Ireland. The immigration guy asked me (presumably hearing my accent) did we plan on visiting "da Nort"? Well we were going to visit Donegal but not actually enter Northern Ireland no (I already knew she needed a UK visa for that).
I asked him why he cared and he explained that the Irish immigration authorities regularly checked that people weren't using Ireland as an easy access route into the UK and were active in preventing it. I wondered why they bothered and he gave me the above answer.
It doesn't fit the agenda of so many here who want to believe that Ireland is the UK's enemy number one, a heaving pit of vituperative bitterness and Anglophobia, but as I have said elsewhere the UK has no better friend than Ireland (and please, don't say the US is the UK's best buddy, it's a laughably pathetic idea) when it comes to quiet behind the scenes cooperation. The UK knows that, that's why the UK has no problem extending its assistance to Ireland whenever necessary, the UK government knows who its friends are beyond the tabloid headlines.
Do you get paid by the word or by the platitude?