Callum played underage level for England before switching to Ireland in 2018 - eligible through his Monaghan-born grandmother.
The fleet-footed attacker featured in the Scott Brown Testimonial but made his Ireland debut against Wales in the UEFA Nations League.
Having made a significant impact at Preston North End, Callum joined Sheffield United ahead of the 2019/20 season and then had a loan spell with West Bromwich Albion.
He scored his first goal for Ireland in the friendly win over New Zealand at Aviva Stadium in 2019. He scored his first hat-trick for Ireland in the 4-0 win over Qatar in October 2021 just days after an impressive brace in a 3-0 win over Azerbaijan.