Quins took a 20-6 lead into the halfway stage at Twickenham, but two yellow cards in the second half almost allowed Irish back into the match.
The visitors eventually held out for a 20-15 victory, but O'Shea was left ruing his side's "ill-discipline" in their opening match of the new season.
"In the first half we were incredibly in control and comfortably the better team but in the second half, we couldn't get our game going at all," he told reporters.
"Our ill-discipline gave them momentum and we gave away a raft of penalties with virtually no decisions going our way.
"But, while I'm not happy with the performance, I'm pleased at the way we dug in when we had players in the bin."