The deceased was discovered by Gardai in an apartment on Achill Road yesterday evening.
The body of a man in his late 20s remains at the apartment, which has been preserved for a technical examination.
An area of Griffith Park in Drumcondra has also been sealed off so forensic invetigators can inspect it.
A man in his 30s has been arrested and is being held at a Garda Station on Dublin's Northside under section 50 of the Criminal Justice Act 2007.
He can be kept there for questioning for up to 7 days.
An incident room has been established at Ballymun Garda Station and investigations are onoing.
