An explosive new documentary by one of Ireland's top investigative reporters has blown the lid of Ireland's oldest and most shocking missing person case. The documentary clearly points the finger at the likely killer and alleges a high level political cover-up to protect him.

In 1977 six-year-old Mary Boyle, a twin, vanished while visiting her grandparents in rural Ballyshannon, County Donegal and was never seen again. She left the remote farmhouse following her uncle who was returning a ladder to a neighbor. The uncle says he ordered her to go back and she has never been seen again.

Two retired policeman testify to the political interference in the shape of a call from a high level politician just as they were closing in on the suspect. They say the intervention froze the investigation and the killer still walks free.

Missing, presumed dead, little Mary is now Ireland's oldest missing person case, and her disappearance is technically still the subject of an ongoing investigation – if you could call it an investigation.

Among those seeking justice for Mary are leading country and western singer Margo O’Donnell, sister of Daniel O’Donnell and a distant relative of the Boyle family who has joined members of the family in their efforts to find justice.

Shockingly, there has never been an inquest, or a commission of inquiry, into the little girl's disappearance and there has been never been a major debate about it in the Irish parliament despite compelling new evidence, including the testimony of the two former Irish police officers who worked closely on the investigation, of an alleged cover-up.

But four decades of apparent inaction may have finally come to an this week after well known investigative reporter Gemma O’Doherty released a deeply disturbing new documentary that contends the six year old was likely killed by someone known to her and that a politically-ordered police cover-up has conspired to protect the killer's identity for decades.

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

As charges go, it's explosive, so it's no surprise that O'Doherty's documentary – which was released on YouTube on Monday but not broadcast by Ireland's national stations – is now closing on 45,000 views.

O'Doherty's documentary alleges that the chief suspect in the unsolved case has never faced justice because of political interference and garda (Irish police) collusion. A mysterious wall of silence descended in the early days of the investigation and it has never lifted, the documentary alleges, with the political interference continuing to this day.

Most shocking of all, the documentary alleges that a child killer is still at large in County Donegal.

Meanwhile Fianna Fail, the main Irish opposition party, have made legal threats against O'Doherty over allegations she has made against its leader Michael Martin and others party members with regard to political interference in the investigation of Boyle's disappearance.

The new documentary adds that although current Taoiseach (Prime Minister) Enda Kenny has met with Ann Doherty, Mary’s surviving twin sister, he has failed to follow up on any commitment to pursue the case.

Ann also alleges to camera that in recent years two gardai (Irish police) made an unannounced visit to her isolated midlands home, intimidating her with their sudden appearance.

RTE, the national broadcaster, did not screen O'Doherty's documentary this week, but they quickly sent the journalist a bill for €13,000 ($14,000) after three and a half minutes of their archival footage was used in O'Doherty's final edit.

This is the sum of money RTE are attempting to charge me for using 3.5 minutes of old archive material on #MaryBoyle pic.twitter.com/SIBf1NocDK — Gemma O'Doherty (@gemmaod1) July 4, 2016

Meanwhile, in dramatic on-camera interviews, two retired local police officers who worked on the case contend they were close to identifying Mary’s killer until their progress was thwarted by the intervention of an unnamed but prominent local politician, resulting in the suspect never being properly questioned.

One of the retired officers, Sgt. Martin Collins, who retired in 1994, states on camera that he believes Mary died within one hour of going missing and that she never left the local area alive. That was his view very shortly into the investigation he said, and almost forty years later his view has not changed.

To date neither the Irish government nor the mainstream media have shown a marked interest in resolving the almost four decades old case.

Watch the documentary here.