Brian's father, Eoin MacNeill was one of the founding fathers of the Irish Free State.
For Brian MacNeill, the Civil War would end with his death in Sligo -- at the hands of the army directed by his father.
In the programme, directed and produced by Niamh Sammon, Michael McDowell investigates what happened that day in September 1922.
He is filmed at Ben Bulben Mountain, Sligo, saying: "This is a place where four young idealistic men met their deaths.
"And it is in a sense a place that evokes their heroism at one level, but also I have to temper that with a knowledge that there were many, many good people who had fallen at their hands on the other side in the Civil War.
"Nonetheless he remains my uncle, and he remains my grandmother's fair-headed boy.
"My grandfather, Eoin MacNeill was one of the founding fathers of the Irish Free State. With Collins, Griffith and WT Cosgrave he grasped the chance the Treaty offered to build a new Ireland. Where my grandfather saw freedom, republicans saw sell out and surrender.
"As Eoin MacNeill sat in Cabinet directing this war, his own son Brian was fighting to destroy the new State."
The programme will be on RTE One on Monday, December 10, at 9.35pm