It took two tries as late as the 64th and 66th minutes for Clongowes to squeeze past Castleknock, 29-19, in a rip-roaring semi-final at Energia Park.
It was tough on the Dublin school, who got off to the perfect start, with captain Louis McDonagh, Conor Delaney and Alex Watson wasting no time in splitting Clongowes open.
Then, the forwards had their say, Luke Callinan pulling in a lineout, Stephen Callinan driving it up and McCarrick carrying the fight for Will Hennessy to touchdown in the seventh minute. McDonagh converted.
McDonagh then powered through for a try which he duly converted. Clongowes clicked and Jack Kearney scarpered clear for an outstanding try in the 18th minute.
Their maul generated the ground and the penalty for David Wilkinson to close the gap to 14-8 in the 28th minute. The longer Clongowes had the ball, the more comfortable Castleknock looked in dealing with it until a high tackle meant an easy position inside the 22 for that menacing maul.
Eventually, Calum Dowling burst over for Wilkinson's conversion to put them 15-14 ahead in the 57th minute.
Jake Rooney flew in at the corner flag in the 59th minute for Knock to go ahead again, but Clongowes answered through replacement Eddie Carr.
Then Oisín Devitt's individual try and Wilkinson's extras made it 29-19 in the 66th minute.