The airline boss racked up over £169,400 when horse Akorakor won at Fairyhouse during the Irish Grand National.
It is believed that O'Leary gave the sum to McNamara, who is paralysed for life following a fall at the Cheltenham Festival.
"Michael phoned from Portugal where he's on a break and instructed that his winnings be donated to JT McNamara," a source told the Irish Sun. "It was a fantastic gesture from him."
McNamara fractured two vertebrae at the first fence in the Kim Muir race last month.