All-rounder Shane Watson took 3-26, including captain William Porterfield with the very first ball, to restrict Ireland to 123-7 at Colombo’s R Premadasa Stadium.
Watson then smashed 51 off 30 balls as Australia, who were runners-up to England in the Caribbean two years ago, reached their target with 4.5 overs remaining.
“We are a better side than that,” said Porterfield, whose men face West Indies next on Monday. “The start wasn’t great, losing those four wickets pretty early.”
Australia meet the Windies on Saturday, and, of Watson, captain George Bailey joked: “He is not bad, is he? His fielding could probably improve a little.”
India also began their campaign with a win after beating Afghanistan by 23 runs. The minnows had threatened at 75-2 after 11 overs, and were chasing 160 for victory, before they were eventually dismissed for 136.