Ireland struggled to find their form for most of the match but eventually did enough to see off the USA in their opening Pool C match in New Plymouth.
The first half was littered with Irish handling errors but they plundered a Tommy Bowe try just before half-time.
James Paterson got the Americans off the mark with a penalty before hooker Rory Best and winger Bowe pounced for quick Irish tries.
Centre Paul Emerick ran through for a late intercept try for the USA.
The wet conditions at Stadium Taranaki made ball handling tricky, but the number of Irish mistakes must be a worry for coach Declan Kidney.
Another concern will be the kicking of fly-half Jonathan Sexton who landed just one penalty from five attempts before being replaced by Ronan O'Gara early in the second half.
The Irish had started the first half on the front foot, pushing the USA back close their line.
However, the momentum was not sustained, and play became scrappy and error-ridden.
Leinster man Sexton split the posts with his third penalty attempt and it remained 3-0 until Bowe went over in the last move of the half.
Bowe, left out of the 2007 squad by Eddie O'Sullivan, took an inside pass from Sexton and dived over for his 17th Irish career try, with Sexton converting.
After Paterson kicked the first points of the second half, Ireland scored tries in the 56th and 59th minutes.
Firstly, Ulster man Best peeled off the back of a maul to charge over, with replacement O'Gara missing the conversion.
Then, rare quick ball for the Irish backline ended in the left corner with Brian O'Driscoll passing back inside for Bowe to get his second try, O'Gara this time adding the extras.
Ireland might have expected to go on and get a bonus point by scoring a fourth try with 20 minutes left.
But they could not find a way through a highly committed American defence, and with the last move of the match, Gordon D'Arcy's loose pass gifted veteran Emerick the chance to streak clear for the USA.
Ireland: Murphy, Bowe, O'Driscoll, D'Arcy, Earls, Sexton, Murray, Court, Best, Ross, O'Callaghan, O'Connell, Ferris, Jennings, Heaslip.
Replacements: O'Gara for Sexton (50), Reddan for Murray (50), Flannery for Best (61), Leamy for Jennings (61), Buckley for Court (65), Trimble for Murphy (68). Not Used: Ryan.
USA: Scully, Ngwenya, Emerick, A. Suniula, Paterson, R Suniula, Petri, MacDonald, Thiel, Pittman, van der Giessen, Smith, Stanfill, Clever, Johnson.
Replacements: Malifa for R. Suniula (60), Moeakiola for MacDonald (63), Usasz for Petri (68), Biller for Thiel (69). Not Used: LaValla, Danahy, Hawley.
Referee: Craig Joubert (South Africa)