It was one of the great art heists of the 20th century, but the motive of the students behind it wasn't to flog the piece on the black market — this was an act of political protest.

On April 14, 1956 two Irish students, Paul Hogan and his mate Billy Fogarty, pinched Berthe Morisot's Jour d'Eté from Britain's Tate Gallery.

The impressionist masterpiece is now worth more than $10 million.

The pair believed the painting was the property of Ireland, and wanted to seize it back in the name of their country.

Berthe Morisot' masterpiece Jour d'Eté is now worth $10 million. ( Wikipedia )

The Lane bequest controversy

Jour d'Eté was part of a priceless collection of 39 artworks known as the Hugh Lane collection.

Hugh Lane had died when the Lusitania, a British ocean liner, was torpedoed by a German U-boat back in 1915.

He had been the director of the National Gallery in Dublin and made it clear in his will that he wanted the paintings to remain in Ireland.

However the document had not been witnessed, and an earlier version of his will had called for the paintings to remain in London.

Sir Hugh Percy Lane's art collection was at the centre of the Lane Bequest controversy. ( Wikipedia )

The British authorities refused to honour Mr Lane's last wishes and 41 years later Hogan and Fogarty decided to correct what they saw as an historical injustice.

Telling the story on the big screen

Two Irish filmmakers, Keith Farrell and Stephen Hogan, are now trying to turn the story of the art heist into a feature film.

Farrell says looking back, the arrogance of the British government was breathtaking.

"If you go back to the records, they say things like while 'the Irish state has a moral right to the paintings. We feel that they are not culturally aware enough to appreciate these work'," Farrell said.

"It was like, 'these dumb micks, how dare they want these paintings back'."

Two Irish film makers Keith Farrell [L] and Stephen Hogan are hoping to turn the Tate gallery heist into a feature film. ( ABC News: Steve Cannane )

Stephen Hogan, who is the nephew of one of the protestors, said his uncle used his status as an art student to get close to Jour d'Ete.

"He managed to get a letter from the director of his course addressed to the head of the National Gallery, asking could he go in and copy a painting," he said.

"And he selected Jour d'Eté because it was such a beautiful and powerful image and he immediately connected with it."

Hogan and Fogarty cased the joint and worked out when the guards took their morning tea breaks.

They decided to strike mid-morning on April 14.

"Paul lifted the picture of the wall, nervously," Stephen Hogan said.

"Because it wouldn't come off easily, and the chap came back … they had to sit down and pretend it was alright."

Paul Hogan and his family found themselves in the spotlight after his photo was published. ( Supplied: Keith Farrell )

When the guard left again the students summoned up their courage for a second time.

"They went for it again," Stephen Hogan said.

"And this time they managed it, they walked down the main corridor of the Tate.

"[And] a chap on the door stepped forward, Paul thought the game was up and said, 'it's alright, it's alright, it's all understood'.

"They never expected to get out that door, but they did."

'The photographer had no idea what he had captured'

Not only had they got the painting out, they had arranged to have a press photographer take a shot of the act.

Captured in that photograph is Hogan in his brother's coat walking out of the Tate Gallery with an impressionist masterpiece under his left arm.

"They'd been smart enough to let the Irish news agency know they were going to do a political protest," Farrell said.

"They booked a photographer and he was standing around and noticing there was no students coming.

"Suddenly this young man is coming down the stairs with a large painting under his arm and behind him someone with an Irish accent went 'take the picture', and he clocked off a couple of frames."

"I don't think he knew what he had until the news broke later that day that one of the Tate Gallery's famous paintings was missing."

News of the theft was published in newspapers all around the world. ( Supplied: Keith Farrell )

The Irish students now had to work out what to do with the painting. They hailed a black cab outside the Tate and asked to go to Piccadilly Circus.

Neither men had been to London before but Hogan had chanced on meeting an Irish woman named Mary the night before.

They headed to her flat and hid the painting under the bed.

Under the spotlight

The artwork was now safe, but Hogan wasn't.

The photograph of him leaving the Tate with a priceless painting under his arm was published in newspapers all around the world.

And as the son of a senior civil servant in Ireland, the spotlight was soon on his family

"When it was discovered that he was the son of Sarsfield Hogan, the you-know-what hit the fan," Farrell said.

"Sarsfield was a typical civil servant, quiet, straight-laced and suddenly he has the press knocking on his door.

"Paul is now a wanted fugitive and it becomes the biggest news story of the day. It was massive."

In the end the fugitives decided to get Hogan's new friend Mary to hand the painting back to the Irish embassy in London — but not before they had passed themselves off as a pair of priests walking around London.

The duo's new friend Mary ended up handing the painting back for them. ( Supplied: Keith Farrell )

The painting was then returned to the gallery.

In the end the protest worked, with the Tate eventually agreeing to share the paintings with Dublin, while controversially retaining ownership of them.

Hogan and Fogarty were let off without charge.

"They were let go unmolested at Liverpool," Stephen Hogan said.

"And a phone call was made I think by DCI McGrath, who was also an Irishman and responsible for pursuing the two lads.

"I think he managed to get the home number and rung up and said, 'he's on the Liverpool ferry, you can pick him up there' and the granny, the mammy, was there in the car and the two lads came off and the only thing she said to Paul was 'get in the car' — no more was said."

The two students believed the painting was the property of Ireland. ( Supplied: Keith Farrell )

Tate Gallery remains unamused

Hogan and Farrell are in the process of getting funding for a feature film on the story of Paul and Billy's act of art restitution.

But they're unlikely to get any money from the Tate Gallery.

Technically the painting is still owned by the Brits and when, a couple of weeks ago, they asked the Tate if they could shoot Hogan sitting in front of the picture in Dublin they were refused permission.

An official told the filmmakers, they were not willing to give permission for the display of their paintings in the context of art theft.

It seems the Tate Gallery remains unamused more than 60 years later.