Chasing a modest target of 124, the Aussies romped home in only 15.1 overs with the opening duo of Watson (51) and David Warner (26) having set the tone by adding 60 runs in the first seven overs in the opening Group B encounter at the R Premadasa Stadium.
Watson's 51 came off 30 balls with help of five boundaries and three huge sixes. By the time he got run out at his team's total on 91, the result had become a mere
Watson had earlier scalped three Ireland wickets for 26 runs -- including the wickets of O'Brien brothers Kevin and Niall -- from his four overs as Australia restricted their opponents to 123 for seven.
The target was never a problem for the formidable Aussies as Watson started the onslaught in the fourth over which was bowled by Trent Johnston. After hitting the pacer for three boundaries, Watson flicked him over deep square leg for his first six.
The second six off Paul Stirling went over deep mid-wicket boundary. But the stand-out shot certainly was the one Watson hit off left-arm spinner George Dockrell to bring up his half century. The batsman rocked back and muscled it into the deep mid-wicket stands. The ball rebounded off the balcony of the upper tier of the stadium.
Watson's 50 came off only 28 balls. He was run-out immediately after that but not before he had taken his side on the verge of victory.
Ireland set Australia 124 to win
Electing to bat, Ireland scored 123 for seven against Australia in the second cricket match of the World Twenty20 at R Premadasa Stadium here today.
Sri Lanka: 123 for 7 in 20 overs (Kevin O'Brien 35, Niall O'Brien 20; Shane Watson 3/26).
Ireland win toss, opt to bat against Australia
Ireland skipper William Porterfield won the toss and elected to bat in the second match of the World Twenty20 at the R Premadasa Stadium, here today.
Australia: David Warner, Shane Watson, Michael Hussey, George Bailey, Cameron White, Glenn Maxwell, Matthew Wade, Daniel Christian, Brad Hogg, Pat Cummins and Mitchell Starc.
Ireland: William Porterfield, Paul Stirling, Ed Joyce, Niall O'Brien, Gary Wilson, Kevin O'Brien, Alex Cusack, Trent Johnston, Nigel Jones, George Dockrell and Boyd Rankin.