Pleascach denied favourite Found to win the Irish 1,000 Guineas at the Curragh.
The 11-2 victor, ridden by Kevin Manning for trainer Jim Bolger, survived a stewards' inquiry.
Pleascach - the Irish word for explosive - won by half a length, with the Aidan O'Brien-trained Found finding trouble in running. Devonshire was third ahead of Jack Naylor.
Found's jockey Ryan Moore was seeking a Guineas quadruple after victories in the three previous 2015 Classics.
Bolger is now set to aim Pleascach at next month's Irish Derby.
Earlier, Al Kazeem (3-1) won the Tattersalls Gold Cup for the second time to clock up a 10th career victor.
James Doyle guided home the seven-year-old, who has been expertly trained by Roger Charlton after returning to racing when a stud career was halted by fertility issues.
|BBC racing correspondent Cornelius Lysaght|
|The stewards' inquiry here was fair enough as Pleascach edged to her left late on, but she always looked safe as there seemed to be no actual contact.|
|Dropping back in distance - so stamina was not going to be an issue - she found reserves when looking likely to be swamped by the fast-finishing Found.|
|Jim Bolger later outlined plans to go for an unprecedented Irish Classic treble in the Derby and then the Oaks later in the summer. Ambitious, but exciting.|