He was replaced after 80 minutes by O'Connor, a club and international teammate of his.
Joe Murphy, United's first choice goalkeeper, was not part of the Republic of Ireland's match squad for the visit of Montenegro.
Murphy's struggle to earn international caps were highlighted by the fact the match marked Shay Given's 100th cap as the Republic's goalkeeper. The Coventry City keeper Kieron Westwood was on the bench.
The three players will be available for Saturday's Championship game against Sheffield United at Glanford Park. They return to the Iron squad on Friday.