RTE's 'A Lost Son' will reveal details of a defining episode in the life of a political dynasty.
In the days following the death of Michael Collins during the Civil War, Eoin MacNeill, a member of the Free State cabinet, said that it was "no civil war – it is a criminal war".
But privately he was hiding a deep family wound.
His second-eldest son, an activist since he was 16 years of age, was fighting on the Anti-Treaty side as second in command of the IRA in Sligo, while two of his other sons, Niall and Torlough, were fighting on the opposite side as soldiers of the Free State Army.
The MacNeills were a family divided.
Now Eoin MacNeill's grandson, the former Attorney General and Justice Minister Michael McDowell SC, has undertaken his own personal voyage to unravel a secret that was never spoken about in the family after what he now calls "the execution" in 1923 of his uncle Brian MacNeill.
His journey to re-enact the circumstances of his uncle's death has taken him from Malahide where the family lived, to a remote cave on the slopes of Ben Bulben, which was used as a hiding place and to where Brian MacNeill was fleeing when he was gunned down by a detachment of the Free State Army.
The resulting investigation has convinced him that his uncle was brutally murdered on the mountain made famous by Yeats and his killing covered up by a government of which his father was a prominent member.
"The MacNeills were a very close family and the bitterness of the time didn't poison their personal lives at all," he says during the personal documentary A Lost Son, which will be broadcast tomorrow night on RTE One.
For McDowell, who was a Fine Gael supporter before founding the Progressive Democrats, following in the footsteps of his rebel uncle was a profound and poignant journey.
He also encountered connections he never knew with former Fianna Fail cabinet minister Mary O'Rourke and former FF TD Jimmy Devins, whose grandfather James was also killed in the same encounter on the slopes of Ben Bulben.
For most people Eoin MacNeill is remembered as the first Commander in Chief of the Irish Volunteers. Originally an associate of Padraig Pearse, the father of seven gradually moved away from what he considered the mis-guided plan for a "blood sacrifice" advocated by Pearse. Instead MacNeill became known in the history books as 'the man who tried to stop the 1916 Rising'.
Deceived by the Irish Republican Brotherhood (IRB) and Pearse, he found out on Holy Thursday that the Rising was going ahead the following Sunday and spent the intervening days trying to stop it, even sending his teenage sons Brian and Niall to Tipperary with orders not to fight.
MacNeill was jailed for life in the weeks following 1916, and the seven MacNeill children were scattered to relatives, with Brian ending up as a boarder in Blackrock College and his brother Niall apprenticed to a Dublin solicitor. After two years in UCD Medical School, during which he was regarded as a brilliant student, Brian dropped out to become a full-time member of the IRA.
As the War of Independence wore on he was dispatched to Sligo to unify and train the local IRA brigade, quickly becoming second in command with the rank of Divisional Adjutant to the OC (Officer Commanding) Billy Pilkington.
In the aftermath of the Treaty he met with his father in Dublin, telling him that he was going back to Sligo to say goodbye to members of the brigade determined to take no part in the impending conflict that became the Civil War. But loyalty to his men won out and he became a leading figure in the Anti-Treaty forces in the west.
Following in his footsteps Michael McDowell visited O'Rahilly House, a large property in Sligo, where he was based and an almost impenetrable cave above Glencar on the side of Ben Bulben where he and his comrades hid during the War of Independence and later the Civil War. He also discovered that Mary O'Rourke's grandmother, Bridget Scanlon, provided a 'safe house' in the district for Republicans on the run.
As the Civil War wore on, the commander of the Free State Army, General Sean McEoin, 'The Blacksmith of Ballinalee' came under increasing pressure to finish off the pockets of resistance in Sligo and Mayo. He sent his personal armoured car, 'The Ballinalee', to assist in the job – but MacNeill and his comrades captured it and laid siege to Sligo itself.
Then, faced with an all out strike by thousands of Free State soldiers, MacNeill fled towards the safety of the mountains. On a ridge on Ben Bulben he and three others were cornered – and the rest remains a mystery that McDowell's legal training tells him was murder.
"I sent my men up the mountain and what happened there nobody knows," said the Free State commander, Tony Lawlor in his version of the incident. Inhumanely the bodies of the four, and two others killed at a separate location, were simply left where they fell and only brought back to Sligo days later by locals. A ring, given to Sinn Fein TD James Devins as a gift by a Sligo jeweller, was even stolen from his finger, and only returned after his widow intervened personally with General McEoin about this "scandalous violation".
Dressed in their Free State uniforms, Brian's two brothers travelled to Sligo to bring his remains back to Dublin, where he was buried at the age of 22 in Kilbarrack Cemetery.
"There was no battle or fire fight," says Michael McDowell, agreeing with the version given by MacNeill's commander Billy Pilkington that the four were "surrounded, disarmed and then murdered".
Eoin MacNeill, by now Minister for Education in the Free State Government, wrote angrily to his son's IRA commander O'Donovan in 1923 accusing him of carrying an "an enormous crime" in sending so many young men to their deaths, influenced by the "artful dodgery of De Valera".
Of his son he said: "He died as he lived, upright, gentle, kind and fearless."
Eoin MacNeill went to his grave believing that his son was shot in the confrontation with Free State soldiers on the mountain side, that as he lay dying he shook hands with his enemies and laughed when they asked him where his comrades were hiding and told them "find them yourself". But Michael McDowell doesn't believe this was the sequence of events.
"I think that a much more accurate version was that he was caught, shook hands with his captors, laughed when they asked him where his men were ... and then they shot him," says McDowell.
While acknowledging that there were outrages on both sides, the Sligo shootings were never investigated and the official version of events, which he investigates 90 years later was "misleading, untrue and riddled with inaccuracies".
When he was told of Brian's death, Eoin MacNeill took his wife Taddy into her study to tell her that her "favourite son" was dead. McDowell's mother told him she could hear weeping "and that was the end of it, it was simply never mentioned again in the house".
"It was an emotional journey for me," said McDowell. "I knew the basic story, but I had never read the letter from Eoin MacNeill which RTE unearthed from the UCD archive. I have sons of the same age as my uncles were in 1923 and that really brought it home to me."
'A Lost Son', presented by Michael McDowell and produced and directed by Niamh Sammon, will be shown on RTE One tomorrow at 9.35pm.