text: He was arrested in Glencree, County Wicklow, in 1957 and sentenced to six months in Mountjoy Prison.  On his release, he was immediately interned in the Curragh prison camp for two years.
  He spent his time in prison studying.  He was particularly inspired by his studies of the Vietnamese struggle for independence. He became a member of the escape committee which engineered the successful escapes of Ruairí Ó Brádaigh and Dáithí Ó Conaill, among others.  Costello would later refer to this time as his "university days".
  Costello eventually reached the rank of Adjutant-General of the IRA. In 1964 Costello was sent to the recently opened Chinese embassy in Paris to seek assistance from the Chinese government.
  Political activism
  After his release, Costello worked to rebuild the republican movement, beginning by building a local base of support in County Wicklow as Sinn Féin's local organiser. Costello strongly supported the movement's left-wing orientation of these years, especially its emphasis on grassroots political activism. He helped found a strong tenants' association in Bray, and also became involved with the credit union movement and various farmers' organisations. During this period, he married a Tipperary woman, Maeliosa, who also became active in the republican movement. Costello was elected to both Bray Urban District Council and Wicklow County Council in 1966.