UCD duo Ben Marshall and David Doyle crossed for tries but hosts Argentina dashed the Ireland Under-20s' hopes of a first win at the IRB Junior World Championship.
Second row Marshall notched a 16th minute try as Allen Clarke's youngsters built an early 13-0 lead.
But a Gonzalo Vila try inspired the Pumitas to get back level for half-time, with Ireland having Paddy Butler sin-binned.
Santiago Mendez's assured goal-kicking helped the hosts move further ahead before replacement hooker Doyle got a late consolation score.
Argentina: Tries: Gonzalo Vila, Tomas De La Vega; Con: Santiago Mendez; Pens: Santiago Mendez 3; Drop: Ignacio Rodriguez Muedra
Ireland: Tries: Ben Marshall, David Doyle; Con: James McKinney; Pens: James McKinney 3