Keenan's first role was in 1999, when she was cast as the teenage girlfriend of Cillian Murphy in the film, Sunburn. After this, she played the character Farrah Phelan in Irish soap opera Fair City for a year. Keenan has stated she would return to Fair City, as another character.
Sinead is 5 feet 1.5 inches (156.2 cm) tall. On 25 September 2011, Keenan announced that she was engaged. Keenan is allergic to chocolate. She married in 2012. Sinead is sister to Actress, Grainne Keenan.
In March 2018, Keenan won the Royal Television Society Programme Award for Actor (Female) for her role as Melanie Jones in Little Boy Blue. In April 2018, it was announced that she had been nominated for a Leading Actress BAFTA TV Award for the same role.
