For quite some time during the Troubles there was a defacto hard border between N. Ireland and everywhere else. If you wanted to drive across the border, fly to anywhere in the UK, or catch the ferry you had to present photo ID of some kind.
I visited the Republic of Ireland a few times in the early 90's (circa 91-92, long before the peace talks and the Good Friday agreement, certainly well before the British army patrolled wearing berets instead of helmets). I travelled by ferry with my car from Scotland (Stranraer) to Belfast (Northern Ireland) and was never, upon boarding or disembarking the ferry, asked to show any ID (I didn't even have a passport back then, all I had was a paper driver's license issued prior to the photo version you get now).
The only place we were stopped was at the British army checkpoint at Newry. Your car entered a concrete "pen" made up of movable concrete barriers. They pointed remote controlled cameras at the occupants and checked your license plate. If all was well you got a green light and drove on your way, again no id required. If you were about to have a bad day you got a red light and an arrow instructing you to drive into a holding area (or a different barrier operated memory is vague on the details) for further checks.
The border with the Republic (on the old road, the R132 now) was unguarded, you just drove on into "the south".
