You still have random stops (or perhaps intelligence initiated?). I was travelling from Belfast to Dublin a few months ago, and saw an immigration patrol pull over a coach. All badged up and obvious who they were.
You may have seen such a stop southbound, northbound checks would be less usual. Many of these are intelligence based as they send someone down to the bus station to see what sort of people are getting on the bus. But US citizens would not be a target of such stops as they don't require a visa. In any case they can only catch the naive.