* Queen visits Belfast for first time in two years
* Voices support for troubled peace process
* Visits set of hit U.S. show Game of Thrones
By Maurice Neil
BELFAST, June 24 (Reuters) - Britain's Queen Elizabeth toured a Belfast prison on Tuesday accompanied by former Irish Republican Army commander Martin McGuinness who was once jailed there, a show of support for the province's troubled peace process.
McGuinness, who was held in the 19th century Crumlin Road prison in the 1970s for membership of the IRA, visited the facility with the Queen whose uncle was killed by Irish nationalists in 1979.
McGuinness is deputy leader of the province under a 1998 power sharing deal which largely ended three decades of violence between Protestants who want to remain British and Catholics favouring unification with Ireland.
His first handshake with the Queen, in 2012, was seen as a landmark in the peace process.
But lingering tensions were exposed in May when Gerry Adams, head of the IRA's political wing Sinn Fein, was held by police for four days for questioning about a 1972 murder, something McGuinness said could stoke dangerous levels of nationalist anger.
A vote by Sinn Fein - McGuinness' party - 18 months ago to restrict the flying of the British flag over Belfast City Hall sparked some of the worst riots for years. Police are now preparing for possible trouble during annual summer marches by pro-British groups.
"I know there are many challenges ahead and peacemaking is not always an easy task, but you have come this far by turning the impossible into the possible," the Queen told a reception at Belfast City Hall after the prison tour.
"The world yearns for examples of positive transformation of people overcoming differences," she said. "I hope and believe that Belfast will continue to be one such living example."
Northern Ireland's pro-British first minister, Peter Robinson, who was himself held at Crumlin Road prison over protests against a deal that gave the Irish Republic a greater say in the running of the province, also accompanied the Queen on her tour.
The imposing sandstone prison is now tourist attraction and conference centre.
The Queen also visited the set of hit U.S. TV series Game of Thrones, one of the highest profile foreign investments in the province since the financial crisis ended a post-peace process boom.
Accompanied by her husband, the Duke of Edinburgh, the Queen browsed the costumes and weapons and met some of the lead actors from the fantasy show made by Time Warner Inc's HBO.
Fans of the series voiced disappointment on Twitter that although she looked at the "Iron Throne", the focus of the series which, according to the plot, was forged from the swords of defeated monarchs, she declined to sit on it. (Writing by Padraic Halpin and Conor Humphries; Editing by Robin Pomeroy)
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