Kearney's songs were highly popular with the Volunteers (which later became the IRA) in the 1913–22 period. Most popular was "The Soldier's Song". Kearney penned the original English lyrics in 1907 and his friend and musical collaborator Patrick Heeney composed the music. The lyrics were published in 1912 and the music in 1916. After 1916 it replaced "God Save Ireland" as the anthem of Irish nationalists. The Irish Free State was established in 1922 and formally adopted the anthem in 1926. Subsequently, theatres and the state broadcaster began playing the anthem at the end of performances, and Kearney prepared to take legal action to demand royalties, obliging the state to acquire the copyright in 1934 for £980 (half each to Kearney and the heirs of Heeney, who had died). From the 1930s, the anthem was increasingly sung in Irish, in a translation by Liam Ó Rinn.
Other well-known songs by Kearney include "Down by the Glenside", "The Tri-coloured Ribbon", "Down by the Liffey Side", "Knockcroghery" (about the village of Knockcroghery) and "Erin Go Bragh" (Erin Go Bragh was the text on the Irish national flag before the adoption of the tricolour).
Kearney was the uncle of the writers Brendan, Brian, and Dominic Behan, both of whom were also republicans and songwriters, via his sister Kathleen Kearney who married Stephen Behan, one of Michael Collins's "Twelve Apostles". Brendan Behan was in prison when Kearney died, and was refused permission to attend his funeral. In a letter to Kearney's son, Pearse, he said, "my Uncle Peadar was the one, outside my own parents, who excited the admiration and love that is friendship."
In 1957 his sister Margaret's son, Seamus de Burca (or Jimmy Bourke), published a biography of Kearney, The Soldier's Song: The Story of Peadar Ó Cearnaigh. In 1976 De Burca also published Kearney's letters to his wife written during his internment in 1921 were published as My Dear Eva ... Letters from Ballykinlar Internment Camp, 1921. A wall plaque on the west side of Dorset Street commemorates his birth there