Almost 40 years after she vanished without trace, the disappearance of six year old Mary Boyle - Ireland's oldest unsolved murder case – will receive a fresh investigation.

Detective Superintendent Walter O’Sullivan of the Irish police force (gardai) told the press on Monday that the investigation will be launched in the next few weeks and will last up to six months.

All the evidence and suspects will be re-examined and the cold case team will also reportedly speak to the serving and retired police officers who originally investigated Mary’s mysterious disappearance.

Mary was last seen by her uncle Gerry Gallagher in Cashelard, near Ballyshannon on March 18, 1977.

Gallagher told investigators at the time that the little girl had followed him as he left her grandparents' home. Gallagher added that he told her to return to the house and did not see her again after that.

Pressure for a new investigation came to a head last week after well known investigative reporter Gemma O’Doherty released a hard-hitting new documentary about the case on YouTube.

O'Doherty's documentary contends Mary was likely killed by someone known to her and that a politically-ordered police cover-up conspired to protect the killer's identity for decades.

On Tuesday the documentary, which hints at the likely killer and alleges a high level political cover-up to protect him, was pulled from YouTube after an unnamed individual threatened a defamation case.

The documentary, which had received 79,339 views since it was uploaded last week, was subsequently removed yesterday and a message on the page gave the following explanation: “The content is not available in this country domain due to a defamation complaint.”

YouTube did not state who had made the allegation of defamation.

The new cold case team will have no pre-conceived ideas of who is or isn’t a suspect and that all evidence will be followed, a police source told the press this week.

The investigation will eventually conclude when a report of their findings is given to the Garda (Police) Commissioner Noirin O’Sullivan.

In O'Doherty's just removed documentary Mary's surviving twin sister Ann Doherty and the prominent country and western singer Margo O'Donnell (a distant relation) express grave reservations about the effectiveness of the Irish justice system in regard to the longstanding case.

Doherty and O'Donnell have spent years attempting to secure the new investigation that the YouTube-released documentary inspired within a week.

But now, with the Irish police effectively probing the prior work of fellow Irish police, some critics wonder how effective the new investigation will ultimately be?

O'Doherty and O'Donnell have grounds to be concerned, they say. In the documentary two retired police officers, visibly haunted by the original investigation, allege on camera that powerful forces interfered with the case.

Two official complaints in relation to Mary Boyle's disappearance were previously investigated by the Garda (Irish Police) Ombudsman Commission (GSOC), with Irish Justice Minister Frances Fitzgerald stating in May of this year that: “I am advised that no misbehavior on the part of members of An Garda Siochana (The Irish Police Force) was identified by GSOC and the complainant was notified of these findings.”

Doherty and O'Donnell were bluntly told that there was no evidence of any of the misbehavior that the two Irish police officers who worked on the original investigation had directly told them had occurred.

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