This has always happened - I've travelled regularly between Dublin and Belfast for 17 years. Couple of times the Gardaà (? I think, Irish officials anyway) got on the Enterprise (Dublin-Belfast train). I had no ID at all on me, but my white face and Irish accent were immediately accepted. Not sure what would've happened to my black colleague with Dublin accent...
I've regularly seen the Aircoach pulled over on the side of the M1 with a police (? again) car beside it, but never been pulled over while on board.
I know international students in Belfast need to get a visa to travel to Dublin, for example for a flight, so I presume the same can be said for plenty of non EU citizens, and that it is violations of those rules that is being looked for.
However, this should absolutely not be confused with a hard border in Brexit discussions.