Watson took three wickets in Ireland's 123 for seven and then struck 51 off 30 balls before Australia romped home in 15.1 overs with seven wickets in hand.
It was an ideal opening game performance for the Australians, who next take on the formidable West Indies Saturday.
Australia put pressure on the Irish from the word go after the latter decided to bat at the R. Premadasa Stadium.
Watson (3-26) got rid of the opposition captain William Porterfield on the first ball of the match and soon after, Ireland were 33 for four in the eighth over.
Niall (20) and Kevin O' Brien (35) steadied the ship with a 52-run stand, helping them to a fighting score.
Australia, however, made short work of the target. Watson and David Warner (26) smashed 60 runs by the seventh over before Cameron White (22 not out) and skipper George Bailey (6 not out) got the team home.
Man of the match Watson tonked three sixes to go with five boundaries.