from "Centenary of Amhrán na bhFiann - The Soldier's Song" by Mícheal Mac Donncha, "An Phobhlacht" 26 July 2007
https://www.anphoblacht.com/contents/17192

"Peadar’s main collaborator in music was his friend and comrade Paddy Heeney, another Northside Dubliner, who put music to Peadar’s words after they started their musical partnership in 1903. Heeney picked out the tunes on his melodeon and then they were noted down.
Some time in 1907, Peadar decided to write a song with a rousing chorus and an unusual metre. The result was The Soldier’s Song. He gave the words to Heeney to put music to them. At first Heeney had trouble with the song and gave up his effort at one point. He persisted and the finished product was first sung by PJ Bourke at 10 Lower Dominick Street to an audience of two – Peadar Kearney and Seán Barlow.

"It is said that at first The Soldier’s Song did not prove popular but eventually so many people asked Peadar for the words that he approached Bulmer Hobson, editor of the IRB newspaper Irish Freedom, to publish them. Thus, the song first appeared in print in 1912.
The now largely forgotten composer of the music to the National Anthem, Paddy Heeney, had died aged only 29 and in poverty in Jervis Street Hospital in June 1911. He was buried in an unmarked grave in Drumcondra Cemetery, where a plaque on the wall now commemorates him. Peadar, then in London with the Abbey Company, took up a collection for his deceased friend’s mother. Among the contributors were two fellow IRB men based in London – Michael Collins and Sam Maguire.

Peadar dedicated his song Slán Libh to Paddy Heeney"

similar information in https://www.jstor.org/stable/30105497 "Peadar Kearney" by Séamus de Burca.