Gardaí investigating the first gangland murder of the year believe a person being questioned last night was one of two men who carried out the shooting on Saturday night.
Detectives are continuing to search for the second man. This man's partner was under arrest last night and was being questioned about the possession of the shotgun used in the murder in Blanchardstown, Dublin.
The victim of the shooting has been named as Dara McCormack (21), Whitestown Park, Blanchardstown.
Gardaí believe his murder was linked to the drugs trade. However, he was not a known criminal.
Detectives are operating on the theory that the dead man owed money to drug dealers, as a result of drug-using rather than dealing. The victim, who lived with his parents, was the subject of a recent knife attack.
The two people detained last night are in their 20s. The man was arrested at 5.30am yesterday during searches in the Blanchardstown area. The woman was arrested at 3am in the Ongar estate, also in west Dublin. They were detained under Section 30 of the Offences Against the State Act and can be held for up to 72 hours without charge.
Gardaí received intelligence about the identity of the killers shortly after the gun attack, enabling them to make the speedy arrests. They had hoped to arrest the second suspect but he was not at home when they raided the Ongar estate. The gunmen are loosely connected to former members of the Blanchardstown-based Westies gang.
Gardaí believe the victim left his family home just before 9.30pm on Saturday and intended to call to a nearby house.
However, a short time later he was approached by his attackers. As he attempted to run from them, he was shot in the back with a shotgun. When Mr McCormack fell to the pavement, his killers fled. They may have left the area by car or motorbike.
Both men wore dark clothing. One was described as being around 6ft while his accomplice was smaller. At least one of them was either masked or wearing a hat.
Mr McCormack, one of seven children, was taken by ambulance to James Connolly Memorial Hospital, Blanchardstown, where he died a short time later.
Garda technical experts conducted a full examination of the scene yesterday. State pathologist Dr Marie Cassidy carried out a postmortem on Mr McCormack, the results of which were expected late last night.
Supt Gabriel McIntyre appealed to anybody who was in the vicinity of the Whitestown estate between 9pm and 10pm on Saturday to come forward. He said gardaí were particularly anxious to speak to anybody who may have seen a car or motorbike travelling at speed in the area around the time of the killing.
Gardaí in Blanchardstown can be contacted on (01) 6667000.
