The NSW shearer was listed as missing in action after the July 1916 battle in France.

Agnes Shannon politely asked the Department of Defence in March 1917 if her brother "is a prisoner of war or is he killed", but there was no news.

Around the same time, German authorities sent Pte Shannon's identification disc to the War Office in London and reported him deceased.

In April 1917, a clergyman officially informed John Shannon his brother had died in the battle of Fromelles, the worst 24 hours in Australia's military history.

Pte Shannon's final resting place remained unknown for more than a century.

He would have remained lost, if not for the army and volunteers trying to identify the 250 Australian soldiers recovered from an unmarked mass grave near Fromelles in 2009, and the involvement of an Irishman tracing his family history as a hobby.

Solving the puzzle of whether Pte Shannon was among those recovered soldiers, who were reburied as 'unknown' in a new military cemetery in 2010, required a DNA match.

The 35-year-old - who worked as a shearer in Merriwa in the Hunter Valley before enlisting - did not have children, and neither did his siblings.

Pte Shannon was born in the small port town of New Ross in County Wexford in southeast Ireland, with his family emigrating to Australia in 1888 after his father died.

Margaret O'Leary from the volunteer group Fromelles Association of Australia spent six years researching all possible branches of Pte Shannon's extended family tree in a bid to find relatives who could provide DNA.

"No matter what we did we just couldn't make it match up," she said.

"When you get back to say 1800 in Ireland the records just aren't there for us to find them."

A family of Shannons in Merriwa was involved in the early search, but the researchers needed to find a man who shared a male Shannon ancestor with the soldier.

Three years ago the association turned to crowdfunding in the hope of paying a professional genealogist in Ireland to continue the search.

A cousin living in New Ross sent a local newspaper article about the search to Dublin-based Patrick Shannon, who like Pte Shannon was born in the port town.

Mr Shannon was intrigued, although he had already traced most of his direct ancestors back to his great-great-grandfather and doubted there was a link to the Australian soldier's family.

"I found it immensely sad to think that a man with the same surname as myself, coming from the same town as myself, had lain forgotten and unknown in an unmarked mass grave for over a century," he said.

He joined Ms O'Leary in researching the soldier's family and gave the army a DNA sample last November.

The DNA link meant Pte Shannon was one of seven soldiers formally identified by the 2019 Fromelles Identification Board.

"It was immensely rewarding to feel that I had contributed in a small way towards the identification of Peter's remains," Mr Shannon said.

"This was tinged with sadness that he had been forgotten for so long and his memory was not known to me or others in the town of his birth, but also pride that he could now be honoured as befitting a fallen hero."

So far, 166 of the 250 soldiers found in the mass grave have been identified.

Ms O'Leary said the army usually required DNA from both the male and female sides of the family, with the initial standard being two samples of each type that were not closely related.

"It's not as simple as Ancestry.com," she said.

"It's very difficult because in 2010 when they did that opening ceremony at the new cemetery, thousands of people had come forward and said 'that's my great uncle'.

"When that all died down the rest of them are very hard to find."

Ms O'Leary said sometimes all the volunteer researchers could hope for was to find a common ancestor going back generations, as happened with Private Shannon.

Even if there were many relatives, finding the particular links needed for a DNA match or enough for an identity to be confirmed could be difficult.

"A lot of them have got DNA that has been donated and it's not a 'no your relative isn't there'," Ms O'Leary said.

"We say to people: don't give up."

Mr Shannon's involvement started as a small gesture on behalf of the town of the soldier's birth.

Private Shannon's name now sits proudly in his family tree.

"He is no longer lost or forgotten."

Australian Associated Press