She was born in Coventry, England. She attended Coventry's Cardinal Wiseman Catholic School and Language College, and studied A-level Drama at Stratford-upon-Avon College between 1996 and 1998. She graduated from RADA in 2003, and made her television debut in the 2004 costume drama He Knew He Was Right. In 2009 she starred in Our Class, a new play by Tadeusz Slobodzianek at the NT and in 2010 Penelope Skinner's play Eigengrau at the Bush Theatre.
On 21 August 2016, Sinead played Hermia/Fairy/Mistress Quince in A Midsummer Night's Dream (Mendelssohn) in the BBC prom 48.
