But, in the end, head coach Nigel Davies was happy to see his side hang on for a share of the spoils as the Dubliners came thundering back into the game.
Fergus McFadden weaved his way over for a try in the right corner after 22 phases of play and then Gordon D'Arcy and the outstanding Sean O'Brien carried the ball from 22 to 22 to pave the way for another touchdown from Shane Horgan.
That left Irish outside half Jonny Sexton with a conversion to level the scores from half way out on the right. It was a kick he would normally have knocked over in his sleep but, having missed 10 points with four successive misses, it was a real test of character.
He held his nerve to hit the mark and make it 17-17 and now it was a case of could the Scarlets hang on. There may only have been two minutes left to play, but it was still anyone's game.
Leinster went back onto the offensive and when Ben Morgan picked up and knocked-on in the same movement at the base of a home scrum in front of the posts on the 22 it provided Sexton with the chance to turn from villain to hero with a drop goal attempt.
O'Brien drove on to make the task a bit easier, but as if to prove it wasn't his night the Leinster No 10 dropped the pass from scrum half Eoin Reddan and the chance went begging.
The Scarlets were left kicking themselves for losing a game they thought they had in the bag, but even worse was the loss of two tight head props, Peter Edwards and Deacon Manu, with suspected broken arms.