Was always referred to as the CTA or Common Travel Area. Agreement between the British and Irish Governments no passport check required so no need for Immigration Officers. It's a bit like a domestic trip with a twist!
UK Special Branch still have a particular interest in arrivals from Eire and can often be found waiting behind desks clearly marked with the Poilice Force crest to check your identity/documents by request. In some airports they may have assumed a 'passive' role where you may be asked to use a channel where you will be asked to provide proof of which flight you've just arrived on by presenting either your boarding card or flight confirmation documentation to a civilian (airport or private security) whether you see or don't see a member of the SP when you arrive they will see you as you will always pass by a camera taking either video or stills, don't expect the camera to be painted bright orange and have a big sign under it saying 'smile please'!