Findings by the Historical Enquiries Team (HET) show that the UVF murdered a Donegal teenager with guns 'stolen' from a UDR barracks.
Henry Cunningham was travelling home to Carndonagh on the Inishowen peninsula with his brothers and workmates when they were ambushed outside Belfast in 1973.
The 16-year-old was killed while his brother Herbert, who was driving the Bedford van, and another Carndonagh man, Packie Doherty, were wounded.
Now his brothers want to know why the Irish government did not make any enquiries on behalf of one of their citizens.
The HET found that the weapons used in the attack were a Sterling SMG taken from the Territorial Army Volunteer Reserve Centre (TAVR)/UDR in Lurgan and a homemade SMG. The ambush took place from a bridge over the M2 near Glengormley.
According to the report two UVF men burst into the guard room and told the soldiers: "Don't worry it's us, the UVF, we'll put them to better use than you."
Declassified documents in the National Archive show that the British Army believed there was strong evidence of collusion in the Lurgan raid. It was one of three raids at the same base in little over a year.
The Cunningham family were not kept up to date with the investigation into their brother's murder and were not told of the arrests of suspects on two occasions and of subsequent releases.
When the homemade SMG used in Henry's murder was recovered, six men — members of the UDA — were arrested and charged under the Firearms Act but not in relation to the murder.
Later in May 1978, two UVF men — one of whom is now dead — were arrested and questioned, but later released.
Henry's brother Robert said: "I hope to hand the report to the new Taoiseach and I hope he will be generous enough to meet us to see what was going on."