Dermot Weld's filly was a narrow winner of the Group Two contest 12 months ago and was 9-1 to repeat the feat on the back of an impressive victory in the Royal Whip here three weeks ago.
Pat Smullen settled Shamreen in front from the off and although the challengers were queuing up early in the straight, with well-backed 5-4 favourite Sea Of Grace travelling particularly well, Shamreen found extra to score by three lengths.
Beautiful Morning and Rain Goddess were second and third, with Sea Of Grace ultimately disappointing.
Weld said: "It was a very good performance by her. I thought she did it very well when she won the Royal Whip here the other day and this was even better today.
"She's won the race two years running and is a very valuable mare.
"I think she's entitled to have a crack at a Group One now so the Prix de l'Opera at Chantilly would be a logical target."
Shamreen's success was a second winner on the weekend for Weld and Smullen following the victory of Eziyra in Saturday's Enterprise Stakes at Leopardstown.
"They are both tough, game, genuine, courageous fillies," the trainer added.
"Eziyra will go for the Group One at Ascot (British Champions Fillies & Mares Stakes, October 21).
"We'll see how the two fillies get on in their next races before we make a decision (on staying in training).
"Shamreen is a four-year-old and doesn't have a lot more to achieve other than a Group One."