Romain Ntamack, son of former French star Emile, led the way as France opened their Six Nations campaign with a win but only after they held off a spirited Irish recovery.
It looked a damage limitation exercise for Ireland when they were blown away in the opening half and trailed by 22-3 at the break as Ntamack punished them.
But Ireland staged a good recovery and will hope to build on that second-half display for the remainder of the Six Nations and for when they return to France in June for the U20 World Cup when the hosts will be in their pool.
A rampant French side got on top at Stade Delmas in Bordeaux and Ntamack gave them the perfect boost with a penalty after three minutes.
The Irish set-piece, particularly the scrum, was solid but basic handling errors in open play were frustrating.
France saturated anything Noel McNamara’s men threw at them and started building a big lead, with Ntamack leading the way in a superb display by the 18-year old.
He struck for the opening try after 13 minutes from a sustained series of surges close to the Irish line when he spotted a gap and slid over for a score which he converted himself for a 10-0 lead.
Harry Byrne, brother of Leinster out-half Ross, pulled back a penalty for Ireland after 22 minutes when the French collapsed a scrum.
France temporarily lost 132kg tighthead Daniel Brennan, son of former Ireland international Trevor, to injury on the half hour but by then they were out of sight.
Flanker Cameron Woki, who has featured for Bordeaux-Begles in the Top14 this season, scored their second try off a lineout in the right corner after Ntamack went to the corner with a penalty under the posts.
Ntamack was unable to land the difficult conversion but he converted his second try after 29 minutes when he sliced through poor Irish defence in midfield to leave them 22-3 in front at the break.
Ireland got back into contention after 54 minutes when full-back Michael Silvestre was taken out by French winger Iban Etcheverry as he chased a grubber from Byrne, with referee Christophe Ridley dishing out a yellow card and awarding a penalty try.
Scrum-half Jonny Stewart gave Ireland hopes of a comeback when he sniped to score in the right corner, with Byrne converting from the touchline to cut the gap to 22-17 going into the final quarter.
But France got a fourth try from replacement scrum-half Jules Gimbert.
Ireland scored a superb try finished by winger James McCarthy, converted by Byrne to reduce the margin to 27-24 four minutes from time.
France wrapped it up with a late try from replacement prop Demba Bamba.
UNDER 20 SIX NATIONS: FRANCE 34 - IRELAND 24
Scorers – France:
R Ntamack 2 tries, pen, 2 cons; C Woki try; J Gimbert try; D Bamba try, L Carbonel con.
H Byrne pen, 2 cons; J Stewart try; J McCarthy try; penalty try
C Laporte; I Etcheverry, P L Barassi, A Vincent, M Marty; R Ntamack (L Carbonel 58), A Coville (J Gimbert 59); H Kolinger, M Lamothe (L Peyresblanques 62), D Brennan (D Bamba 30-41, 57); K Geraci, T Lavault; C Woki (B Heguy 41), I Diallo (A Roussel 63), M Lemardelet.
M Silvester; P Sullivan (A Kernohan 56-63-72), D McCarthy, T O’Brien, J McCarthy; H Byrne, J Stewart (H O’Sullivan 63); J Duggan (J French 74), R Kelleher (D Barron 71), J Aungier (T O’Toole 55); C Daly (R Coffey 68), J Dunne; M Dalton, A Hall (S Masterson 41), J O’Sullivan.
C Ridley (England).
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