Derek Halligan is a Northern Irish actor. He has many stage credits, and is best-known on television for a recurring role in Hollyoaks. He played the supervillain Dr. Nightshade in Tonight We Fly.
In Game of Thrones Halligan was originally announced as playing the role of Ser Alliser Thorne. He was confirmed in the role on 16 July 2010. On 18 August 2010 it was announced that Halligan had left the project for unspecified reasons. He was replaced by Owen Teale.