The scrum-half was punched in the face by a provincial rival during a fracas, the Herald can reveal.
However, he has decided not to make any complaint to gardai about the alleged incident which occurred on Sunday, November 4.
Cooney (22) required medical treatment after he was hit by a "random punch" in a southside Dublin club.
The alleged assailant is a current Connacht rugby star.
Cooney was part of the Leinster squad that travelled to Swansea to take on the Ospreys on November 4, before the international break, where they lost.
The players not involved in Declan Kidney's plans were given the week off and Cooney and a few friends went socialising in south Dublin.
"It wasn't even a row, he was punched. Thankfully he's fine," a Leinster spokesperson told the Herald.
"It occurred nine days ago after the Ospreys game, the squad who weren't involved in international duty had the week off and it's obviously one of these things that happens. It's unfortunate but the most important thing is that he's fine and he reported for training with the rest of them yesterday.
"It's not very nice when it happens. It wasn't a row, fisticuffs, guards anything like that. It was just a random punch and he was just the poor unfortunate at the end of it.
"There are no criminal charges going to be pressed."
Connacht rugby chief executive Tom Sears admitted that he was aware of the alleged incident but had not received any complaints.