Amhrán na bhFiann (The Soldier's Song) was written in 1907 by Peadar Kearney, an uncle of Brendan Behan, but was not widely known until it was sung both at the GPO during the Easter Rising of 1916 and later at various camps where republicans were interned. Soon after, it was adopted as the national anthem, replacing God Save Ireland. The first edition of the song was published only in 1916.
The same song with english words is here.
