He was first elected to the Northern Ireland Assembly in 2003 and was re-elected in 2007 as a Sinn Féin member for the East Londonderry constituency. He resigned in 2010. In 2005, he was arrested by the Police Service of Northern Ireland (PSNI) and questioned about a bombing incident in the village of Claudy in 1972. An official complaint was lodged with the Police Ombudsman regarding the lawfulness of the arrest and this investigation is ongoing.
Brolly mounted a legal challenge in relation to the arrest against the PSNI, which settled out of court.
Brolly is an Irish culture enthusiast. A lyricist and musician, he has written many well-known Irish republican songs, most famously The H-Block Song.
He served as the Assembly spokesperson for Sinn Féin on Culture, Arts and Leisure, sitting on that Department"s oversight committee. He is also an Irish language enthusiast and is a fluent speaker of the language.