"Irish beauty Saoirse Ronan is dating her “Mary Queen of Scots” co-star Jack Lowden, Page Six has exclusively learned. The 24-year-old three-time Oscar nominee Ronan turned up with hunky Lowden, 28, at a celebrity screening of their new movie Tuesday night, and they looked extremely close at the after-party at Tavern on the Green, sources said. “Jack was following her all over,” said a source. “He even followed her to the bathroom and was waiting outside for her. They were very close.” Another source said that the pair were joined at the party by Ronan’s mom, Monica, before Ronan had to head for a flight back to the set of Greta Gerwig’s in-production “Little Women,” which wraps in a week and also stars Emma Watson, Meryl Streep and Timothée Chalamet. In “Mary Queen of Scots,” Lowden — who previously starred in “Dunkirk” — plays Ronan’s on-screen husband Lord Darnley, a bisexual cheater with an unfortunate fate. Off-screen, Ronan was at first believed to be dating Lowden’s ballet dancer brother Calum after they met at the BAFTAs this year. But more recently, eagle-eyed fans have spotted Ronan with Jack on vacation in the Dominican Republic and other spots. A movie source told us of Ronan and Jack: “We don’t know when they got together, we only found out on Tuesday night when they rolled up together — but they seem very happy and sweet.” Before filming, Lowden said he was excited to work with Ronan since “she’s lovely . . . I can’t wait for that it’s going to be fun.”