Niall studied piano and composition at the Royal Irish Academy of Music. He was awarded first prize at the 1992 Dublin Film Festival's music competition, chaired by composer Michael Nyman. Since then he has composed music for films, animation and television drama.  

Niall has received several IFTA (Irish Film & Television Awards) nominations and won the award for his score to the BBC film Loving Miss Hatto, which told the story of concert pianist Joyce Hatto. In 2017 he received the Venus award at the Badalona Film Festival in Barcelona for his score to the animated film The Lost Letter, narrated by Kate Winslet. His orchestral score for the classic Irish silent film Guests of the Nation, which was commissioned by the Irish Film Institute, has been performed by the RTE Concert Orchestra in Dublin (National Concert Hall), New York (Lincoln Center) and London (Barbican Centre).  

Niall also writes piano music and has released three solo piano albums. Two of these albums The Promenade and Still Life were released by the contemporary classical label 1631 Recordings. His third piano album Nightsongs is published independently.  

Recent film credits include the psychological thriller In Darkness starring Natalie Dormer. Recent television dramas include the BAFTA award-winning ITV drama series Little Boy Blue, the ITV drama series Hatton Garden and the critically acclaimed UK drama series ManhuntManhunt premiered in January 2019 and was ITV's highest rated new drama since the launch of Broadchurch in 2013.