Harrington, 36, won British Open despite hitting into the water twice on the 18th hole of regulation. He beat Sergio Garcia in the subsequent four-hole playoff.
Harrington, from Dublin, Ireland, also won the Irish Open in a playoff.
"In my eyes The Irish Open is the fifth major," said Harrington. "It's my national event and when I get there it's incredibly hyped up and there's loads of expectation and pressure. So it mimics a major for me."
He won the Order of Merit title in 2006 was finished third on those standings in 2007 with winnings of about $3.6 million.