Johnny came through the underage system at his native Sligo Rovers, playing for their U15s, U17s and U19s. The forward was included in Liam Buckley’s senior squad in 2021 and made his league debut against Dundalk at the Showgrounds in March. Johnny scored 12 goals for the Bit O’Red, including one in their Europa Conference League tie against FH Hafnarfjordur of Iceland. He also made his Ireland Under-19 international debut later that year. In January 2022, Johnny left Sligo to sign for Celtic in a five-year deal. First team opportunities were limited in Scotland, with the 19-year-old making 12 appearances on loan at Championship club Queen’s Park.