The Special Detective Unit (SDU) (Irish: Aonad Speisialta Bleachtaireachta) is the main domestic security agency of the Garda Síochána, the national police force of Ireland, under the aegis of the Crime & Security Branch (CSB). It is the primary counter-terrorism and counter-espionage investigative unit in the country. The Special Detective Unit superseded the Special Branch (which they are still commonly referred to as), which itself replaced the older Criminal Investigation Department (CID), which was founded in 1921. They work in conjunction with the Defence Forces Directorate of Military Intelligence – Ireland's national intelligence service – on internal matters. The unit's headquarters are based in Harcourt Street, Dublin City.