Sabrina Carpenter and Barry Keoghan are keeping it casual. Amid rumors of their split, a source exclusively tells PEOPLE that the relationship between Carpenter, 25, and Keoghan, 31, is “on and off.” On Friday, Aug. 16, Keoghan also seemingly responded to the breakup rumors by liking the “Please Please Please” singer’s flirty Instagram post, which showed her posing in baby …