The painting, by artist Colin Davidson, was unveiled at an event in London on Thursday attended by the Co Clare-born novelist and Ireland’s ambassador to the UK, Adrian O’Neill.
Speaking at the event, O’Brien recalled some of the experiences when she sat for Davidson at her London home last September, saying it had brought back memories from 80 years ago.
She added: “I felt I was being stripped but stripped in a good way that would be necessary to what would be eventually on the painting.
“As a writer I never want to lose my past and never to lose my earliest feelings and impressions and fears and all else.”
O’Brien emigrated to London in 1958. Her first novel, The Country Girls, was published two years later in 1960.
She established an international reputation as a novelist, short story writer, memoirist, poet and playwright.
In a 2012 review of her memoir, former president Mary Robinson referred to her as one of the great creative writers of her generation.
O’Brien continues to live in London and celebrated her 90th birthday last December.
Speaking at Thursday’s event, Mr O’Neill said he was “delighted” the portrait will have its permanent home at the embassy.
He said: “In years to come I am sure visitors to the embassy will gaze on the portrait and be delighted that the Country Girl has returned home.”
Davidson said he hopes the portrait captures some sense of spirit and courage of a “remarkable woman”.
Addressing O’Brien, he said: “You are very much loved and appreciated. You have written modern Irish history in your own way and we thank you for that.”
The portrait is 50×46 inches, similar in size to Davidson’s portraits of Seamus Heaney, President Michael Higgins, John Hume, Christy Moore, Britain’s Queen Elizabeth and Brad Pitt.
Davidson is also the creator of the Silent Testimony series, individual portraits of 18 people who experienced loss during Northern Ireland’s Troubles.