The former Fianna Fail minister's memoir Just Mary has shifted 5,684 copies since its release last month.
It recounts the 'Mammy' of the Dail's political career as well as the grief at the death of her nephew Brian Lenihan.
Her closest rival is chick-lit writer Amy Huberman, whose latest offering I Wished For You has sold 3,177 copies.
And Ms O'Rourke celebrated last night after picking up the John Murray Show Listeners' Choice Award at the Bord Gais Energy Irish Book Awards.
It was a night for the women as the late Maeve Binchy was give a posthumous prize for Popular Fiction, while Tana French won Crime Fiction Book of the Year.
And among those turning heads were boxer Katie Taylor in leopard print dress and Cecelia Ahern in a blue shift dress.
According to figures from Nielsen Bookscan, Olympic gold medallist Katie's book My Olympic Dream has sold 2,149, including 1,729 last week alone.
Total sales for Melanie Verwoerd's When We Dance were (2,903), Lar Corbett's All in My Head (1,919) and presenter Bill O'Herlihy's We'll Leave it There So (984) show they are also popular choices.
Twink's book Unzipped has only shifted 166 copies.