https://www.ipoi.gov.ie/en/types-of-ip/copyright1/understanding-copyright/duration-of-copyright-protection/
the Irish copyright office states:
Literary, dramatic, musical and artistic works and original databases: Copyright protection expires 70 years after the death of the author/creator.
O'Neill died in 1963 so it will become public domain in 2033.   
However, it seems he used the melody of an older Irish tune, "Banks of the Murlough Side" (according to https://www.irishcentral.com/roots/history/foggy-dew-song-history) so perhaps the music is public domain...