Enright's book beat out works by Ian McEwan and Lloyd Jones, which were favored to win.
Also on the shortlist for the $100,000 prize were Mohsin Hamid, Nicola Barker and Indra Sinha.
"The Gathering," about an Irish woman prompted by her brother's suicide to revisit three generations of her dysfunctional family's history, is the fourth novel written by the former television producer.
"'The Gathering' is an unflinching look at a grieving family in tough and striking language," Howard Davies, chair of judges, told the BBC.
"We think she is an impressive novelist, we expect to hear a lot more from her. The book is powerful. ... It has one of the best last sentences of any novel I have ever read."
"I am still churning it through," Enright told BBC Radio 4. "Tomorrow, I'll wake up and go 'whoopee.' "I was ready for anything -- possibly anything except that."
Enright, 45, also said she wasn't offended by Davies' remarks that her book was "depressing" and "a little bleak" in places.
"They're entirely fair. It's not a cheerful book," she said.