If you have ever been lucky enough to be embroiled in a late night pub lock-in in Ireland, then you will know that they are amazing craic and should be regarded as cultural phenomenons.
They are also one of those Irish things that you forget not everyone in the world actually knows what they are.
So when Jimmy Fallon essentially described being involved in a lock-in in Kinsale to Saoirse Ronan, she totally knew what he was on about.
However, when she explained the concept out loud, it did actually sound a bit mad.
"The whole idea of a lock-in was that the guards- our policemen- wouldn't know about it," the 21-year-old very aptly explained.
"But they're usually in the lock-ins as well. They're usually in there with you.
"I've grown up with that really."
Watch the hilarious clip below: