Saoirse Ronan was on the show to promote her new movie 'Mary Queen of Scots,' which releases in The States this week. She was on-hand to sing a rendition of The Pogues and Kirsty MacColl's classic Christmas song 'Fairytale of New York'.
Now, before you give it a watch, we have to give you a brief warning; Jimmy Fallon attempts a raspy Dublin accent, not too far off Shane MacGowan... but it's actually not that bad. In fact, (dare we say it) it's actually quite good.
Saoirse does a great job too, and they even get a chance to sup on some watery American Guinness. Awh bless.
And if that wasn't enough Saoirse for you, she also turned up for a chat with Jimmy on his sofa.
A mega-fan of the movie 'Bridesmaids' (who isn't?), Saoirse's trivial knowledge was put to the test with a series of questions only a true super-fan would know.
Saoirse's new movie 'Mary Queen of Scots' releases in Ireland on 18th January. Saoirse's co-star Margot Robbie also turned up to down some shots with Jimmy recently too.