It happened after a row broke out in the building on New Wapping Street – which was being used by squatters – shortly before 11 o'clock.
Two people were injured – the man in his early 40's died at the scene, whilst another man in his late 20's was taken to the Mater Hospital with non-life threatening injuries.
A man in his mid 30's was arrested a short time later and is being held at Store Street Garda Station.
The scene remains sealed off for a technical examination and a post mortem will be carried out later.