Taking the anti-Treaty side in the Irish Civil War, Derrig took part in fighting against National forces in Dublin.

He escaped from Dublin on 30 June 1922 and served as Adjutant to Ernest O'Malley during fighting in counties Wicklow and Wexford in July and August.

Derrig was appointed IRA Adjutant General by Liam Lynch in November 1922 and was serving in that position when arrested by National Forces on 6 April 1923.

[6] On that same date, while in custody of the Criminal Investigation Department in Oriel House, Derrig was shot in the face by a CID detective and lost his left eye.

Derrig initiated a review of industrial and reformatory schools and the rules under the Children Act 1908 (8 Edw.

It has been suggested that he did not want to follow British law reforms in the 1920s and 1930s, because of his strong anti-British views, and that Irish children had suffered needlessly as a result.