On loan from Premier League side West Ham United, Cullen has been named in Ireland's squad for their UEFA U21 Championship qualifier against Serbia.
The match will take place this Friday (7 October 2016) in Waterford, Ireland.
Cullen has already featured for Ireland's U21s this season, playing in their last two qualifiers.
The midfielder's latest call-up means he will now miss City's Checkatrade Trophy tie with Bury and the Sky Bet League One match against Shrewsbury Town.