Fellow Lion Jonathan Sexton's conversion made it 16-10 and, with Ian Madigan also going over later in the half, Ireland secured victory despite a late score for the hosts Argentina.
Zebo was called up as injury cover for Tommy Bowe for the Lions' successful tour of Australia this time last year and Ireland head coach Schmidt admitted he discovered a lot in Argentina.
"We've achieved an opportunity where we broaden the group a little bit, we've found out a little bit about some of those players, some who've acquitted themselves really well and some others who've still got some work to do," he said.
"That's an achievement in itself. At the same time we know we have got a heck of a lot of work to do. There are some teams that will put us under a lot more pressure than that.
"I think, again our set-piece probably saved us. We put them under scrum pressure a few times when they had great platforms and when they did get that platform in the last play of the game we weren't able to defend it when they did get the ball in their hands, which is massively disappointing."