Argentina, who had never won away to Ireland in 11 attempts, took the lead on 10 minutes but the hosts responded quickly and comprehensively as two tries from fullback Keenan either side of a Dan Sheehan effort put them 21-5 up by halftime.
Sam Prendergast swiftly sealed a bonus point with his first international try before Keenan and Ronan Kelleher made it a six-try win. Mateo Carreras' second try of the game was all the thoroughly outclassed visitors could muster in response.
Ireland began the weekend in fourth place in the tightly contested northern section of the new tournament and face home games against Fiji and South Africa before the finals matches at Twickenham. Argentina next travel to Italy and France.