After my daughter interned at the EU for her study abroad a few years back, a group of the interns went to the UK and Ireland before coming back.
Most of them had their plane reservations to leave from Shannon with them when they got there. One smart ass kid from Philly (imagine that) didn’t have his yet and was picking up a wire of money from his parents. When asked his visit purpose he said to get drunk in the pubs. Bad answer. When they found he had no money on him to get out of the country and no plane ticket or reservations, he was detained until his parents contacted the consulate over here and sprang him. He was a guest of the Garda for at least 24 hours. No open border there.