The Welsh region had taken a maximum 15-point haul from their first three games thanks to bonus-point triumphs over Zebre, Connacht and Treviso.
But they gave themselves little chance of making it four from four in Dublin as an ill-disciplined firt-half performance left them 24-0 adrift at the RDS.
Having fallen 31 points down early in the second half, the visitors rallied through tries from Ben John, Dafydd Howells and James King, with Dan Biggar adding four points.
But it was not enough as they eventually suffered a 31-19 defeat. Tandy was disappointed with the display and is keen for the Ospreys to get their season back on track as soon as possible, starting with the upcoming trip to face leaders Ulster in Belfast.
“It wasn’t about a lack of effort,” said Tandy. “That was there but some of the detail was out, some of the discipline, and we made it difficult for ourselves. It does not get any easier with Ulster coming up next week.
“We knew it was a strange start to the season with two home games and three away trips to Ireland. We have won one of the two Irish trips so we cannot be too downhearted, it’s still a solid start to the season but as we have said all along that is all it has been, a start.
“We will be positive going to Ulster but well aware we will need to be much, much better than we were against Leinster right from the first whistle.”
