There is no official ruling on the above circumstance, it comes down to common sense.
Me personally in a situation like that would have taken several factors into consideration, the state of the pitch and the possibilty of further injury. The extra long delay in the ambulance arriving, the demeanor of both sets of players (nobody likes to see injuries like this), if possible consult both captains or managers.
The competition being played and the score at the time. Munster and National fixtures are on a tight schedule and are supposed to be finished on the day, this was probably the over riding factor in the Ref's decision to continue the game, though if it was me in my day I would have abondoned the game, simply because of the time delay in getting the player to hospital and as the incident happened so early in the game, the possible risk of further injury to other players due to the state of the pitch. I always believed that the players safety was paramomount.