Ahern earned one cap and scored two tries for Ireland under-18 clubs against Portugal under-18s in November 2017, and won two caps for Ireland under-19s in their capped friendlies against France under-19s in April 2019. One month later, Ahern was selected in the Ireland under-20s squad for the 2019 World Rugby Under 20 Championship, his first call up at that level internationally. He made his under-20s debut when he started in the third pool fixture against Italy on 12 June 2019. Ireland won 38–14, though Ahern left the field injured during the first half. He recovered in time to start in the 30–23 win against England on 17 June and the 40–17 defeat against New Zealand in the 7th-8th place play-off on 22 June.