A tender and rarely seen portrait of a boy by Lucian Freud that hung in a legendary Irish house tucked away in the Wicklow mountains for more than 50 years is to appear at auction for the first time.

The 1956 painting, Head of a Boy, is of Garech Browne, the wealthy Guinness heir who became a patron of Irish arts and music and hosted wild, dazzling, parties at the fairytale-esque Guinness home and estate.

Browne was 16 when he sat for the 34-year-old Freud at Luggala. The result was “a jewel of a painting”, according to Tom Eddison, a contemporary art specialist at Sotheby’s inLondon.

“It is funny with Freud, you don’t really get a grip of how good he was until you see these things in person, especially these works from the 1950s which you don’t see very often.

“It is so precise and so beautifully executed. It is a really extraordinary painting, a very tender and beautiful portrait,” he said.

Freud first visited Luggala with his then wife, Kitty Garman, in the 1940s when Browne’s mother, the socialite Oonagh Guinness, reigned supreme. Freud and Garman were to divorce and the artist married into the family, eloping to Paris with Browne’s cousin Lady Caroline Blackwood in 1953.

Facebook Twitter Pinterest Garech Browne in the 1950s. He became a patron of Irish arts and music. Photograph: Dorian Browne/Sotheby's

Freud and the young Browne were kindred spirits, with the artist sneaking him in to Soho drinking holes. He introduced him to figures such as Francis Bacon, who became a lifelong friend, and took him on an educative trip round the Louvre in Paris.

As an adult, Browne became an important patron of the Irish arts, setting up Claddagh Records in 1959, which championed the band The Chieftains as well as poets including Seamus Heaney.

Parties at Luggala, an 18th-century gothic-inspired hunting lodge, were the stuff of legend. They attracted Hollywood actors, politicians, poets, artists, rock stars and hangers-on. The Beatles took acid there; Kofi Annan enjoyed country walks in the 2,000-hectare (5,000-acre) estate. Other names in the Luggala guestbook include Brendan Behan, Marianne Faithfull, Mick Jagger, John Hurt, Dennis Hopper, Charlotte Rampling and Michael Jackson, who reportedly used Luggala as a hideaway for six weeks in 2006, soon after being acquitted of child molestation charges.

Browne, the last custodian of Luggala, died in London last March, aged 78.

Facebook Twitter Pinterest Luggala. The fairytale-esque Guinness home and estate, the scene of wild, dazzling parties. Photograph: © Dorian Browne via Sotheby's

The portrait is well known but little seen. It was included in Freud’s blockbuster 1974 retrospective at London’s Hayward Gallery and a 2008 show in Dublin but otherwise has hung in Luggala’s sitting room for more than 50 years.

“It is so wonderful,” said Eddison. “If you’re a Freud geek like me, when you know of these paintings but never really see them … when you see them for the first time it is such a powerful thing.”

The work is Freud’s second attempt at painting Browne after the first was damaged in a fire at Luggala.

At 18cm by 18cm, it is a small, intense painting, similar in size to Boy Smoking (1950-51), in the Tate collection; and Freud’s 1952 painting of Bacon that was in the Tate collection but stolen while on display in Berlin in 1988. Remarkably, it has yet to be recovered.

Head of a Boy will be sold at Sotheby’s contemporary art sale on 5 March with an estimate of £4.5m-£6.5m.