But Ms O'Rourke has not applied for the tax exemption available to artists living in Ireland.
The exemption excludes artists and writers from paying tax on the proceeds of original works, though a cap of €40,000 was placed on the measure in 2011.
But Ms O'Rourke added that her income from the sales will not be anywhere near the overall amount made.
"How could it be? You don't print it up and sell it yourself off the side of the road," the ex- Fianna Fail TD said.
Launched in October, the book recounts Ms O'Rourke's decades-long political career, as well as her grief at the death of nephew Brian Lenihan.
Speaking to the Herald, she said the memoir has been "selling steadily since it was issued", adding "All I can say is, and rather immodestly, is that I think it's a good story."
"I don't know how much I'll earn because the book has not completed its sales yet," she said.
"I didn't apply for the tax relief. I knew nothing about it. This is a whole new field for me.
"The only way I knew anything about the tax was an article during the week," Ms O'Rourke added.
Among the books Mary's tome outsold were chick-lit writer Amy Huberman's I Wished For You and Melanie Verwoerd's When We Dance.
"I want to say thank you to all who bought it," she said.
Ms O'Rourke says she enjoyed the experience of writing so much that she intends to do it again.
Instead of typing out the pages, she dictated the memoir into a tape recorder.
"I loved it, the process of writing it ... I do miss the talking. I'm better at talking than anything else.
"It's a talking book -- I talked it into my tape recorder," she told RTE Radio.
The retired politician is thinking about ideas for a second book, but it will definitely be non-fiction, saying "real life is far racier".
Since it was published, Just Mary -- A Memoir has racked up sales of more than 12,000 copies in Ireland.
In the book, Ms O'Rourke writes movingly about her final conversation with former Finance Minister Mr Lenihan before he passed away from cancer in 2011.
Her sales of 12,423 up to Christmas were ahead of former President Mary Robinson's memoir, Everything Matters, which sold 9,175.
Former presidential candidate Senator David Norris's autobiography, A Kick Against The Pricks, racked up sales of 2,171.
New figures show some 55 individuals claimed just over €20m in artists' exemption in 2010, compared with 33 who claimed €25.6m the previous year.