Hollywood actor Martin Sheen has declared himself proud of his uncle's IRA past.

The star said he was also relieved to discover that his mother's brother, Michael Fieland, from Co Tipperary, had no part in the assassination of Irish revolutionary leader Michael Collins.

Sheen found out about his close family links to Ireland's War of Independence while taking part in the US version of the hit genealogy television series Who Do You Think You Are?.

During the making of the programme, the 71-year-old visited Dublin's Kilmainham Gaol and spent time in the cell where it is believed his uncle was incarcerated.

"I'm enormously proud of him," he said. "I would like to hope that if I had been here in Ireland at the time, I would have followed him. And I would have been as committed as he was."

Best known for his roles in Apocalypse Now, Wall Street and the television series West Wing, he described his uncle as an Irish volunteer.

The actor said Fieland went on to fight against the Free State side, who supported the Anglo-Irish treaty, during the resulting Civil War in the early 1920s.

Sheen admitted he was concerned about what would turn up during the trawl through his family past.

"When I was in Ireland and discovering the involvement of my uncle in the Rising and the Civil War, because he took an opposing side to (Eamon) de Valera, I was afraid he might have been in on the plan to assassinate Mick Collins," he said.

"But as it turned out he was in prison when Mick Collins was assassinated and I was deeply relieved."

Known for his social activism, Sheen added: "I have been involved in a lot of campaigns for peace and social justice and I had the same kind of commitment in those areas that he had here."

One of 10 children, Sheen, whose real name is Ramon Estevez, was born to a Spanish father and Irish mother, Mary-Anne Fieland, from Borrisokane in Co Tipperary.

She emigrated to the US during the War of Independence, and he believes she was sent away to protect her from the intense hostilities.

"And she was meant to come back when the fighting stopped and the Republic was established in 1923," he said.

"And so it was a very, very satisfying moment for me to know that she, too, was involved."

During the programme, Sheen also discovers a relative on his Spanish side who was a social justice activist wrongfully jailed during Franco's fascist regime.

The father of screen actors Charlie Sheen and Emilio Estevez describes how both his Irish and Spanish relatives had risen up against oppression and compares their actions with his own political activism.

"You don't expect to change the world, you don't expect to even influence your family or your friends," he said.

"You do it because you cannot not do it and be who you are or who you are meant to be."

Sheen fulfilled a life dream by returning to his mother's homeland to study in 2006 when he took a place as a mature student at the National University of Ireland in Galway.

Sheen said the fact his mother learned Gaelic indicated to him that she was also involved in the struggle for independence.

"I learned while I was in Ireland that the British officers were very, very reluctant to search young girls under the age of 16 and so often they were used as couriers with information supporting the Rising," he said.

"And so they also had to speak Gaelic because in that way they could speak right in front of the Black and Tan or any of the British soldiers there and not be understood and she spoke Gaelic."

He added: "And so to have learned it on her own was a very informative bit of information that confirmed that she was involved and that she was sent away for her own protection.

"That was the first [thing] that hit me in the eyes and made me realise she was as committed as her brother, Michael and risked as much."

Describing his Irish and Spanish uncles as "absolutely heroic", Sheen said he believes activism may be part of his DNA.