On December 19, 1991, Galbraith was born in Kent, UK to Siobhan and Alec Galbraith. His father is an Englander and his mother is an Irish. He has a younger sister Bernadette. His best friend is his elder cousin Joe.
When Galbraith was a young boy, his grandfather, who played several musical instruments in a band, took Galbraith to fleadhs (concerts). The mix of Scottish and Irish musical traditions.
Galbraith insisted on performing spontaneously at the annual Rochester Dickens Festival, a two-day extravaganza with his sister Ayannah where people were invited to dress up in Victorian costumes to celebrate the life and times of the famous author Charles Dickens and his links with their town. Galbraith, dressed up as a chimney sweep, started singing. Soon after this he started to enter local talent contests and within a year he had won 15 titles and more than £1,000.
As a result of his success in the talent shows, the major recording companies soon heard about Galbraith and he was signed to his first recording contract in England. His first recording was "Walking in the Air", which was released on a special Christmas Hits album, also featuring songs by Westlife, Elton John and Elvis Presley. Among some of his most memorable performances have been the Queen's Jubilee at St Paul's Cathedral, when he sang "Amazing Grace" accompanied by the St Paul's Choir and a performance in front of more than 22,000 people at an Elton John concert. His first self-titled album, Declan, with Irish traditional songs as well as specially written material became a big success and charted in the U.K. and Ireland. Within the year of release, it had soon sold 200,000 copies in Germany.
Another big success came during a nationwide tour of Young Voices concerts when breaking the Guinness World Record. It was at the Odyssey Arena, Belfast in December 2002 when he sang live with some 10,000 children and was also simultaneously linked, by radio and satellite, with more than 80,000 children in their schools all over the UK, who accompanied him in achieving the world’s largest choral sing. His talent and reputation came to the attention of media mogul Haim Saban who signed Galbraith to his music group's new record label, Starwatch, and selected Germany as the first site to launch their first album. Galbraith parted company with Starwatch in February 2010 and is reportedly battling major contractual issues that prevent any releases for the time being.
For some considerable time Declan have worked on his music career again. In 2014 he was planning a comeback. He has started to present his new, self-composed songs in small clubs. Currently he is working in the studio.