The bitter irony for coach Justin Burnell was that Wales had also beaten Australia, France and England earlier in the season.
"It has been a big learning curve for us," said Burnell. "We came here with plenty of confidence but our losses have made us rethink strategy and selection. We will go away from here and learn from our experiences."
Wales started with a bang, scoring two tries in eight minutes through flanker Dan Franks (St David's College) and hooker Ken Owens (Carmarthen Quins) - his fourth of the championships to make him Wales's top try scorer.
Ireland remained ultra competitive for the rest of the half and got a vital try back early in the second half through prop Cian Healy. Fly half Brian Collins added the points and soon did the same to a try from replacement prop James Gethings.
Ireland, their tails up, flew into the Welsh in furious defence and frenzied attack with the Welsh boys affected more by the hot conditions. A Collins penalty stretched the lead to 17-10 before No.8 Lewis Evans (Ebbw Vale) bravely clawed a try back with five minutes left.
To their credit, Ireland finished the stronger with a drop goal from Collins bringing the curtain down on a bad day for Wales.
Tries: D Franks, K Owens, L Evans
Ireland U 19
Tries C Healy, J Gethings
Cons B Collins (2)
Pen B Collins
Drop Goal B Collins
Martyn Thomas (UWIC/Scarlets) (rep Rhys Jones (Sale/Exiles) 29'); Alec Jenkins (Carmarthen Quins/Scarlets), Ashley Smith (Newport HSOB/Dragons), Gareth Maule (Newport/St Alban's RCS/Dragons) (rep Luke Ford (St David's College/Blues) 43'), Jimmy Norris (Bettws HS/ Newport HSOB/Dragoms); Rhys Priestland (Carmarthen Quins/Scarlets), Darren Allinson (Pontyberem/Scarlets); Hugh Gustafson (UWIC/Scarlets), Ken Owens (Carm Quins/Scarlets) (rep Marc Jones (Sale/Blues) 49'), James Corsi (Pontypool/Bassaleg CS/Dragons), Bradley Davies, (Cardiff/Blues) (capt), Lou Reed (Llanelli/Coleg Sir Gar/Scarlets), Matthew larsen (Tonyrefail CS/Blues), Lewis Evans (Ebbw Vale/Dragons), Dan Franks (St David's Catholic College/Blues) Referee Keith Brown (New Zealand)
IRB U19 World Championship: Final day Friday April 21
Championship final New Zealand 13 Australia 17
Play off for places:
3rd /4th England 12 France 12 (England are 3rd on tries)
5 and 6 Wales 15 Ireland 20
7 and 8 Argentina 26 South Africa 7
9 and 10 Scotland 15 Samoa 12
11 and 12 Japan 13 Romania 13 (Japan finished 11th after a penalty shoot out, Romania are relegated to be replaced by Section "B" winners Fiji)
02 New Zealand
08 South Africa