Cullen played the whole match as Ireland defeated their Slovenian visitors 2-0 at Waterford in their UEFA 21 European Championship Qualifier thanks to late goals from Harry Charsley and Sean Maguire.
Cullen actually set-up Charsley's opener three minutes from time with a header from Jack Byrne's cross before Maguire's penalty on the stroke of full time sealed the win.
The 20 year-old West Ham United loanee and the rest of his Irish team-mates will now turn their attentions to their next qualifier against Serbia.
Ireland U21s are due to play away to Serbia next Tuesday (6 September) before Cullen is scheduled to return to City training.