Since joining in August 2023 after leaving European heavyweights AC Milan, Heffernan has established himself as a regular within United's under-21s squad, captaining the young Magpies on a number of occasions in 2023-24.

The teenage centre-back made his senior debut at the age of 16 for hometown club Cork City in October 2021 and, after joining Milan on loan for the second-half of the 2021/22 campaign, made a permanent switch to the San Siro the following summer.

After completing a successful trial with United's under-21 side during the young Magpies' 2023/24 pre-season campaign, Heffernan put pen to paper on a professional deal with United and made his under-21s debut for Newcastle's second-string during a Premier League 2 clash against Reading.