A man in his 40s was being questioned last evening at Shankill Garda Station in Dublin in connection with the death of a 12-year-old boy in Killiney.
The boy's body was discovered at his home in Bayview Rise, Killiney, at around 6.30 p.m.
The man was detained at Shankill Garda Station under Section 4 of the criminal justice act.
The boy's body was kept at the scene until late last evening while the Garda technical bureau carried out its examination of the scene.
The house is situated in a quiet cul de sac between Killiney and Shankill.
Last night, shocked neighbours gathered in groups in front of the semi-detached house.
Some of the local children were prevented by their parents from talking to the media, many of them reacting angrily when the TV cameras turned on their lights. Those who did speak said it was a "deep, personal tragedy."
The State Pathologist, Dr Moira Cassidy, will carry out a post mortem examination on the body this morning.