His paternal grandfather was also a solicitor and supporter of Charles Stewart Parnell, James J. Walsh, who represented two of the Invincibles found guilty of assassinating Lord Frederick Cavendish and Thomas Henry Burke.
He attended Belvedere College, Dublin, until the death of his father in 1934, after which his mother could no longer afford the tuition and he became a boarder in Newbridge, County Kildare.
[1][3] He was unable to attend university due to the family's financial situation, and when he left boarding school he joined the Local Defence Force.
While still serving in the army, he wrote an article defending Irish neutrality for the New York Herald Tribune under the pen name Rex Mac Gall.
In 1949, the family moved to Dublin, where they were part of a group which established the first gaelscoil, Irish language school, Scoil Lorcáin in Blackrock Town Hall.
[1][3] He attended night classes in Irish at Trinity College Dublin (TCD) from 1958 to 1960, where he was taught by David Greene and Máirtín Ó Cadhain.
Breatnach also composed music, including a work dedicated to those who died on Bloody Sunday, and "An Ghailseach" inspired by his wife.
He was an advocate for the use of Irish in all aspects of life, including from his motor insurance company, leading to a long running disagreement over the forms and documentation provided only in English.
Many Irish language organisations supported him, and after he was jailed for a number days in April 1966 in Mountjoy Prison, his fine was paid by a donor.
During the 50 anniversary of Easter Rising in 1966, he joined 12 fellow members of the Irish-language activist group Misneach on a hunger strike to highlight the failure of the Irish state to accomplish the goals of the 1916 proclamation.
The group picketed the GPO, Liberty Hall and the new Garden of Remembrance, and their protest was covered by both national and international media.
[1][3] Alongside his wife, Breatnach was an advocate for human rights globally, particularly opposed the authoritarian regimes in South America.