Originally from Wexford town, where his father was an optician and jeweller on Main Street, he came to Dublin in the late 1960s. Around 1970 he was a member of the poetry-and-music group Tara Telephone, in which he composed, sang, and played guitar. He and poet/percussionist Eamon Carr left Tara Telephone to form the Celtic Rock band Horslips, which Sinnott left in 1972, before the recording of Horslips' first album in 1973. He married Kathy Sinnott (née Kelly) when he was twenty two. They had nine children together, but are now separated. One of their children, Kevin Sinnott accidentally drowned while swimming at university in Georgia, US on 21 September 2009.
In the 1980s, he was a member of Moving Hearts. He then devoted himself to production work with both Mary Black and her sister Frances Black. During a 13-year and six-album association, he toured all over the world with Mary Black. He has been working with Christy Moore for almost 30 years, since appearing on Christy Moore's Ride On album in 1984.
He plays and tours with Christy Moore. At concerts Sinnott has performed "Corrina, Corrina" as well as "St. Louis Blues" and "Lord Franklin". He played to a full house in Wexfords Arts Centre on his homecoming gig in July 2012, one night after his sell out show with Christy in Galway. This was his last gig prior to the release of his first solo album of original works.
He recorded primarily original songs with Owen O'Brien, I Love The Noise It Makes, released on 7 September 2012 in Ireland, and on 10 and 11 September in the UK and US respectively. The first single was "Sunshine In".
This was followed up in 2015 with the album, Window On The World.
- "Declan Sinnott: Guitar star hits the right chords". Enniscorthy Guardian. 6 May 2009.
- "Tributes as Sinnott's son is laid to rest". Irish Examiner. 1 October 2009. Retrieved 10 August 2013.
- "Declan Sinnott launches first solo album". RTE. 10 September 2012. Archived from the original on 18 January 2013. Retrieved 12 February 2020.
- "Declan Sinnott, I Love The Noise It Makes, CD review". Telegraph. Retrieved 10 August 2013.
- Declan Sinnott's facebook page
- Declan Sinnott home page
- Listen to the Irish Times preview of the album (Ireland only)