One of said fans was 5-month-old baby James Barr and his father (who should probably be a journalist), Alan Barr.
The duke and duchess last visited Northern Ireland in 2016.
Can you see Kate and William having another baby? I think William might be slightly anxious'. The Duchess, who wore a powder blue coat by Mulberry for the outing - stopped to chat with a nine-year-old girl who is working on a biography about her.
They are visiting Belfast, Fermanagh and Ballymena on the two-day trip which focuses on young people.
Prince William appeared to take the challenge seriously, tilting the pint glass and watching the tap intently before exclaiming "Look at that bad boy!" as he admired his work.
As part of their visit to Ballymena, the couple also visited St Joseph's SureStart centre in the Dunclug area of the town, which supports parents with children aged under four years.
The couple were there to learn more about the work of Cinemagic - a charity that uses film, TV and digital technologies to inspire and educate young people.
Britain's Prince William, Duke of Cambridge and Britain's Catherine, Duchess of Cambridge poor a pint of Harp Ice beer during their visit to the Empire Music Hall in Belfast, Northern Ireland on February 27, 2019.
The couple arrived to a warm reception on Tuesday morning before visiting the Windsor Park Football Stadium where they mingled and showed some sport skills with participants in the Irish FA's "Shooting Stars" programme.