Ó Lionáird was born and raised in Baile Mhúirne in the West Cork Gaeltacht, a primarily Irish-speaking region.

[2] Ó Lionáird earned a Bachelor of Education degree at Carysfort College in Dublin and was employed as a primary school teacher for seven years before making a career as a singer.

He has said that hearing Nusrat Fateh Ali Khan singing on Peter Gabriel's album Passion was a "light bulb moment" which made him think that sean-nós could also "inhabit this ambient soundscape".

He went on to write a major work, Anáil Dé (The Breath of God) for Ó Lionáird and members of the Crash Ensemble.

Ó Lionáird suggested the title and the texts for the piece, which is based on a collection of Irish prayers and poems.

[14] Ó Lionáird sang the piece at Carnegie Hall in May 2013 in a concert of Dennehy's music with the Crash Ensemble and Dawn Upshaw.

[13] Along with fiddlers Martin Hayes and Caoimhín Ó Raghallaigh, guitarist Dennis Cahill, and pianist Doveman, Ó Lionáird is a member of The Gloaming, an Irish-American supergroup whose self-titled first album was released in 2014, winning the Meteor Choice Music Prize for Irish album of the year.

[18] In 2013, University College Cork appointed Ó Lionáird the School of Music and Theatre's first Traditional Artist in Residence.

Ó Lionáird plays Frankie Doran, a poor workingman who sings the traditional song "Casadh an tSúgáin" (Twisting the Rope) during Christmas dinner at a soup kitchen.

[21] In May 2015, Ó Lionáird hosted a five-part radio documentary series about singing entitled Vocal Chords, which drew on "personal experience, academic contributions and a global cast of vocalists" including Sinéad O'Connor and Christy Moore.