The murder of Mary Boyle: Irish cold case team will pursue Ireland's oldest missing person case.

A documentary to be released this month will highlight the unsolved case of Ireland’s youngest missing child, Mary Boyle.

Written and directed by journalist Gemma O’Doherty, the documentary hears from Boyle’s twin sister, Anne Doherty; her neighbor, the singer Margo O’Donnell; several Gardaí who worked in the area at the time; and human rights lawyer Darragh Mackin, who is working with Doherty to find justice for her sister.

Ann Doherty’s battle for the truth began when her 6-year-old twin sister, Mary Boyle, went missing on her grandparents' remote farm in Cashelard, Co. Donegal. Her remains have never been found.

Doherty believes Mary was sexually assaulted and murdered by someone she knew and that her body was dumped close to where she went missing. She also believes she knows who is responsible for the horrible crime but that political intervention early in the police investigation attempted to cover the killer’s tracks, allowing the perpetrator to walk free.

Throughout the documentary, Gardaí working in the rural Donegal area when Mary went missing speak of interviews with a man they knew to be her killer, but that the investigation was halted when a local politician called the Garda station to intervene in his questioning. As such, the man many in the locality believe they know to have committed the crime has never been arrested.

Earlier this year, Doherty traveled to the US to meet with Congressmen and other politicians to further highlight the case, and Congressman Brendan Boyle, whose father is from Co. Donegal, pledged his support.

While in D.C. she also met with senior politicians from Northern Ireland including Secretary of State Theresa Villiers, First Minister Arlene Foster, Deputy First Minister Martin McGuinness, and Nigel Dodds DUP MP for North Belfast, as well as former congressman and human rights lawyer Bruce Morrison and Fr. Sean McManus of the Irish National Caucus.

This week Doherty penned an open letter to Ireland’s Ambassador to the US Anne Anderson, asking for her assistance in raising knowledge of the case among the Irish American community.

“You might remember we met in March at your event for St Patrick's Day,” she wrote, “My twin sister is Mary Boyle - Ireland's youngest missing person.”

“She was six when she was sexually assaulted and murdered in Donegal in March 1977 and her killer is still at large.

“He has been shielded by a politician who is still in public office and who contacted the gardai after her murder and told them not to arrest him. He never has been.

“I would be very grateful if you could assist me in any way you can to get justice for Mary, and raise her case among the broader Irish American community,” she continued.

Read more: US Congressman pledges support for family of Ireland’s longest missing child

Ann Doherty with Catherine Murphy TD and AnneMarie McNally of Soc Dems.

Ann Doherty with Catherine Murphy TD and AnneMarie McNally of Soc Dems.

Last week, Boyle’s case was heard in the European Parliament and Ann Doherty has previously spoken on her sister’s behalf in the parliaments of Stormont, Brussels and Westminster. Despite these meetings, Doherty feels she is still faced with reluctance to undertake a proper investigation into her sister’s death.

“It is appalling that I have to leave Ireland to try to get justice for my sister,” Doherty previously told IrishCentral.