Members of the unit are specially trained and selected Detective Gardaí who are tasked to remain covert whilst on and off duty, tracking suspected criminals, terrorists and hostile, foreign spies operating in Ireland.

The National Surveillance Unit is understood to possess a manpower of approximately 100 officers, and is considered to be the most secretive arm of the force.

The unit is headed by a Detective Chief Superintendent, who is under the supervision of the Assistant Commissioner in charge of the Crime and Security Branch.

It is understood to have a strength of about 100 plainclothes officers, mainly detectives, who all carry firearms (standard issue is the concealable Walther P99c 9mm semi-automatic pistol).

Increasingly, the NSU is using technical and electronic espionage rather than physical and human intelligence, working with the SDU and Directorate of Military Intelligence (G2) of the Irish Defence Forces, mainly to counter terror, militant and subversive groups in Ireland,[4] as well as serious crime (particularly drug trafficking) and the activities of hostile foreign governments on Irish soil (counterintelligence).