His is one of the most famous faces of the first World War, captured fleetingly in a historic film of 1916. He was long thought to be English, the personification of the brave “Tommy”, and to have died later in the war.

But the mystery soldier carrying a wounded comrade through trenches at the Battle of the Somme may instead have been a Dubliner, from a family of Finglas Republicans, who later turned down a military pension, played GAA for a club he had helped found, and lived until 1982.

Now, 102 years after the events depicted, descendants of Charlie Brennan believe they are close to proving he is the man in the film. And the Imperial War Museum in London agrees they have a strong case, boosted by comparative analysis of the footage and a photograph of Brennan playing football for Erin’s Isle a few years later.

The past century has brought an estimated 100 different claims from people who think the trench man is an ancestor. But the Brennan family’s case in now in the top “two or three”, according to an IWM spokesman. It is hoped there may be fragments of evidence somewhere that may yet clinch the argument.

Shot by English cinematographers Geoffrey Malins and John McDowell, the Somme documentary was first planned as a propaganda film depicting what was hoped would be a quick, dramatic victory in the great offensive of July 1916.

As the campaign became a bloody stalemate instead, the film’s narrative changed too. Now it depicted the bravery of British soldiers under fire, dramatising the need for more recruits.

Many in its audiences had never seen a movie before, and the scenes of dead and dying soldiers must have been shocking. Even so, the film was seen by 20 million people in six weeks, a record unmatched for decades afterwards.

John Brennan suspects a historic reluctance in Britain to accept his ancestor as the face in the film, because he didn’t fit the narrative It was long part of Brennan family history that “Uncle Charlie” was the hero of the trench scene. He had returned from war with a Distinguished Conduct Medal, and his souvenirs also included a spiked German helmet which, back on the family landholding in North Dublin, was used to “collect eggs”.

But as John Brennan, a grandnephew, explains, Charlie’s official war record proved elusive. He had never claimed a pension. Even his regiment was uncertain. Worse still, the DCM medal had at some point been lent to a work colleague in Unidare and never returned.

All that remained were memories. John, now 75, recalls his mother’s uncle as a character, a man who “drank for Ireland” and would sing “16 verses” of a song at every family function. It was Brennan tradition that when sailing to the war from Dublin’s North Wall, he had even serenaded well-wishers from the deck.

This may explain how he became a fleeting film star. John guesses that he had already come to the attention of Geoffrey Malins in an earlier short film depicting Irish soldiers at the Somme, and that Malins chose to feature him in the main documentary, in a scene partly staged.

But more recently, harder evidence has begun to resurface. Military records at last unearthed a Charles Brennan, Soldier 48408, from “Finglay (sic) Dublin”, who had served with the Royal Garrison Artillery, which was indeed present in Beaumont Hamel, the film location, on the date in question.

Technical analysis of the 1923 GAA photograph has supported the case for Brennan as the man in the film. The analysis couldn’t prove this in the positive, but it would have exposed any major anomalies in facial characteristics. No such anomaly was found.

The head of the IWM’s film archive, Matthew Lee, is used to encountering certainty from relatives attributing heroism to ancestors. But Brennan’s case goes well beyond “family mythology”, he agrees, and makes him “a very strong candidate”, in the “top two or three” .

John Brennan suspects a historic reluctance in Britain to accept his ancestor as the face in the film, because he didn’t fit the narrative. Not only was he Irish, he had brothers in the 1920s IRA. In other respects too, he was not the ideal hero.

One of the lesser mysteries long surrounding him was that, having been promoted to corporal, he ended the war a mere gunner. But the IWM may have explained this too. Lee notes the existence of a court martial record from 1918 for one “C. Brennan”, on charges of being absent without leave and “violence to superiors”.