A man in his 30s and a teenage boy were in Garda custody last night on suspicion of shooting a 16-year-old in an attack that has left the victim fighting for his life.
Gardaí said the injured boy, who is from Swords, Dublin, where the attack happened, was "completely innocent".
They believe he was shot after his attackers mistook him for somebody else.
The juvenile suspected of carrying out the shooting is himself aged 16. He was arrested with a 36-year-old man at the scene of the attack at St Cronin's Close, Swords, at 8.20pm on Saturday.
The victim was walking in the estate with three friends, all locals, towards nearby River Valley Park when his group was approached by the attackers. One of the attackers shouted something at the four boys and opened fire with a shotgun.
The victim was wounded twice in the upper body. He was rushed by ambulance to Beaumont Hospital where his condition was critical but stable last night, according to Garda sources.
Detectives do not believe the wounded boy or any of his three friends was the intended victim of the attack. Gardaí are working on the theory that the attackers were looking for another boy with whom the 16-year-old, being held last night, had earlier been in dispute over damage to a motorbike.
Garda sources said a gun attack of that nature, by one 16-year-old on another intended as revenge over a minor personal matter, was without precedent in their experience.
Two other people, a man and a woman in their 30s, were also arrested in Swords in connection with the incident. They were being detained last night on suspicion of withholding information.
The man and teenager arrested were being held for questioning in relation to possessing the shotgun used in the attack. The woman being held is the partner of the 36-year-old arrested at the scene.
All four were being questioned under Section 30 of the Offences Against the State Act. They were being questioned at Garda stations in Swords, Malahide, Whitehall and Santry. They can be held for up to 72 hours without charge.