James Cullen first became interested in Northern Ireland and the Troubles when he visited in 1969 during a leave of absence from the US army while the country was embroiled in the Vietnam war.

Born in New York to a Sligo father and Offaly mother, he was drafted into the US army as a private after he completed law school. He was appointed to the judge advocate general's (JAG) corps, the US military's justice system, as an army lawyer. He was later promoted to become a brigadier general, a one-star general and, eventually, chief judge of the US army court of criminal appeals.

Mr Cullen (70) retired from the army in 1996 and now works as a commercial property lawyer with New York law firm Anderson Kill.

Travelling around Northern Ireland in 1969 with a fellow US soldier originally from Co Cavan, he saw how local people had been mistreated by the RUC in Coalisland, Co Tyrone. He saw parallels with how African-Americans were treated in Georgia in the American south when he was stationed there as a young soldier around the same time.

“I saw, most importantly for me, how people were being treated, how the RUC had beaten the hell out of them,” said Mr Cullen. “The sense of helplessness of the people who were beaten” was what he recalled most from his time in Northern Ireland, he said.

My Lai massacre

During his time in the US army he provided legal representation to the chaplain in the investigation whether there had been a cover-up of the My Lai massacre of hundreds of Vietnamese villagers by US soldiers in March 1968.

During the 1980s he was asked by the late prominent New York lawyer Paul O'Dwyer, a native of Bohola, Co Mayo, and Mr O'Dwyer's nephew Frank Durkan to help defend IRA men fight extradition cases in the US. To ease the workload on the late Mr O'Dwyer's law firm, the lawyers set up the Brehon Law Society in 1976 to take on pro- bono cases. Mr Cullen was the first president of the society.

The New Yorker helped out in the defence of IRA member Joe Doherty in his nine-year fight to stop his extradition and deportation from the US to Northern Ireland.

He also assisted in the defence of six men who were acquitted in the 1990s of conspiring to acquire weapons for the IRA in a case in Tucson, Arizona, providing bail for one of them.

“We had a wide network of lawyers and sometimes in unlikely places,” he said, noting the support of local lawyers in Arizona.

Hosted Haughey

Mr Cullen was, along with Paul O'Dwyer, asked to help Fianna Fáil raise funds in the US in the late 1980s. They hosted Charlie Haughey at a dinner at the prestigious Starlight Roof in the Waldorf-Astoria Hotel on New York's Park Avenue in April 1988. They later withdrew their support for Haughey after he said one thing to the Irish-Americans about his policy on Northern Ireland at the dinner and another to the Irish media on his return home.

A member of the board of advisers of human rights group, Human Rights First, Mr Cullen was one of a group of retired generals and admirals who spoke out publicly against the administration of president George W Bush on the use of torture and interrogation and who have called for the closure of the Guantánamo prison in a US naval base in Cuba.

Mr Cullen has investigated the 1989 loyalist murder of Belfast solicitor Pat Finucane testifying on the killing before the US Congress. He has also assisted in the inquiry into the killing of another Northern Irish solicitor, Rosemary Nelson, by loyalists in 1999 and has acted as an international observer of Orange Order marches in Northern Ireland.

"Jim Cullen is one of the most respected guys in the Irish-American community, not only for his advocacy but for his integrity," said New York lawyer Brian O'Dwyer, a well-known figure in the city's Irish-American community.

The retired brigadier general is critical of the British government’s refusal to hold an inquiry into Finucane’s killing and of the Irish Government’s failure to support calls for an inquiry.

He believes the British have a responsibility to disclose publicly details of state collusion in loyalist killings of Catholics in Northern Ireland through the Troubles, which, for him, justified the IRA’s military campaign.

“This is where the state allowed itself to become so corrupt. Who could swear allegiance to it?” he said. “That’s why I had no problem with armed resistance to that kind of government.”

Mr Cullen became president of Friends of Sinn Féin in 2012, succeeding another New York lawyer Larry Downes.

Sharing the same goals as Sinn Féin, Friends of Sinn Féin, according to Mr Cullen, wants to see “reunification of the country, to see it put back on an economic basis where people don’t have to emigrate, where there is fairness and transparency in Ireland”.