When Alec Cobbe was growing up in a mansion near Donabate, the painting was just one of many in his family drawing room.
The handsome Tudor gentleman with a bulky lace collar and a somewhat inscrutable mouth gazed out over the family as they went about their business.
"As a painting it didn't really stand out, because there were an awful lot of paintings in the house,'' Alec Cobbe told me this week.
Alec grew up in a time warp at Newbridge House, one of the finest Georgian mansions in the country. There was no electricity and there was no running water. The house was lit with oil lamps.
Life had remained more or less the same at Newbridge since his ancestors were leading landlords in the area in the 19th century. The Cobbes were considered benevolent, and went to great lengths to help poor families during the Famine, but their fortunes had faded by the middle of the last century.
Down through the generations, Alec's family thought that the man with the insightful demeanour in the painting was Sir Walter Raleigh, the famous explorer and courtier of Elizabeth 1.
For centuries the picture remained at Newbridge House, largely unnoticed, a face in a crowd of family portraits and landscapes. But in the past few days it has caused a sensation that has rippled right across the literary world.
The Shakespeare scholar, Professor Stanley Wells, has come forward to claim that the inconspicuous work from Donabate is the only surviving portrait of William Shakespeare, painted from life.
If we are to believe the professor, this is the only picture of the Bard as he really was. For the first time, it is claimed, we can gaze into the soul of the man who wrote the soliloquys of Hamlet, Romeo and Juliet, and the love sonnets.
Forty-six when he posed for an unknown painter, Shakespeare looks well-to-do, somewhat dashing, and, in modern fashion, his wrinkles may have had a bit of air-brushing.
If the identification is correct it alters what we know about the playwright.
It depicts him as a man of wealth and high social status, undermining the claims of some conspiracy theorists that he lacked the refinement to write the great works attributed to him.
After putting the painting on display in Shakespeare's hometown, Stratford-on-Avon, Alec Cobbe hopes to bring it back to Ireland for exhibition in the near future.
"There will be some issues with insurance to be sorted out, but I hope to bring it back to Newbridge,'' he said.
The Cobbe family were influential aristocrats who moved to Dublin in the early 18th century. One self-effacing Cobbe, the writer Frances, famously said of her family that it was "chiefly remarkable for never having done anything remarkable''. But she was wildly wrong on that score.
Frances herself was one of the leading campaigners for women's rights in the 19th century. Charles Cobbe, the man who built Newbridge House, was Archbishop of Dublin.
And now the Cobbe family may have offered an insight into the most famous figure in English literature.
So how did the portrait come to be in the hands of a leading Anglo-Irish family, and how did it go unnoticed?
Over the past few decades, Alec, himself an art restorer, has been carrying out detective work into the unusual paintings that were hanging for centuries at Newbridge. By exploring the origins of the paintings, Alec had already established a link with Shakespeare before the latest find.
Among the other hidden treasures, Alec found a portrait of a person who appeared to be a young woman with smooth skin, long hair and rosy cheeks.
The painting was thought to be of Lady Norton, daughter of the Bishop of Winton. But Alec, with the help of researchers, established that the young lady was not, in fact, a woman at all.
Sensationally, he claimed the painting showed the Earl of Southampton, a nobleman linked to the Cobbes. The Earl was Shakespeare's only known literary patron.
There has been much speculation that the Earl and the writer had an intimate bisexual relationship. The playwright dedicated two of his poems to Southampton.
The Earl is also thought to have been the "fair youth'' of Shakespeare's sonnets, and possibly the "master-mistress of my passion'' mentioned in one of the poems.
The Earl is now thought to have commissioned the portrait of Shakespeare. According to the latest research, it came to the Cobbes when one of the family married a great granddaughter of the nobleman.
When the Cobbes moved to Ireland, the portrait of the androgynous earl and the portrait of his friend, the playwright, are believed to have gone with them.
Alec Cobbe says: "I have no doubt that the Shakespeare picture went over to Ireland when the family went there. The confusion arose, because Archbishop Cobbe later scribbled Walter Raleigh on the back of it. That is why it was thought to be Raleigh.''
Cobbe's theory that the portrait was the Bard was spurred by a visit to the Searching for Shakespeare exhibition at the National Portrait Gallery in London in 2006.
One of the works on display, which usually hangs in the Folger Shakespeare Library, Washington, caught his eye.
This painting had for centuries been regarded as a bona fide portrait of Shakespeare, but was discredited in the 1940s when X-rays showed that the sitter's hair had been painted over to correspond with traditional images of the writer.
Cobbe saw the cleaned-up version, with the hair restored, and realised that it was a copy of his mystery painting at home back in Ireland. He called Professor Wells.
The professor, general editor of the Oxford Shakespeare and chairman of the Shakespeare Birthplace Trust, originally had doubts.
"I am a scholar and a sceptic," he says. "But over the years my excitement has mounted. I am willing to go 90pc of the way to declaring my confidence in the identification."
To support the claim, experts carried out a barrage of scientific tests. These included tree-ring dating of the oak panel on which the portrait is painted, X-ray analysis and infra-red photography.
At the time of the portrait, Shakespeare would have been at the height of his powers, fresh from the romance of Antony and Cleopatra and about to plunge into the fairytale worlds of The Winter's Tale, Cymbeline and The Tempest.
Professor Wells now believes that most previous candidates for a realistic likeness of Shakespeare were inferior corruptions and some were copies of the Cobbe portrait. They were made after the writer's death.
Professor Wells said this week: "The evidence that it represents Shakespeare and that it was done from life, though it is circumstantial, is in my view overwhelming. I feel in little doubt that this is a portrait of Shakespeare, done from life and commissioned by the Earl of Southampton.''
The find will help to stoke up speculation about the playwright's private life and his relationship with the earl.
It suggests that he had a longer-lasting bond than previously thought with the Earl, one of the most flamboyant noblemen of the era.
The Cobbes lived at Newbridge House right up to the mid-eighties, but now spend most of their time in England.
The house and its grounds are now run by Fingal County Council, but the Cobbe family still has access to the house. They maintain an apartment on the top floor.
The discovery has certainly caused excitement in Donabate. Local historian Peadar Bates, who has written extensively about the Cobbe family, was stunned when he learned about the painting's origin on the news.
"I have seen the painting several times in the Red Drawing Room at Newbridge House, and it was always thought to be Walter Raleigh,'' he said. "It is an amazing discovery.''