He closed with a round of 72 to win the title by two shots over Ballybunion's Peter Sheehan.
The 65-year-old player from Greystones finished with an impressive nine straight pars to withstand the challenge from Sheehan.
"It's tremendous," said O'Leary. "I've been playing these seniors events for ten years.
"I've had two seconds and a third up to now, so I'm thrilled to bits.
"I was quite nervous. I feel a mixture of joy and relief but I'm over the moon."
He was four strokes clear going into the final day, but a triple bogey at the third halved his advantage but he withstood that early wobble to hold his nerve.
Birdies at five and six for the leader repaired most of the damage.
"I was still in the lead and that was the most important thing," he said.
