Victim found on Fairview Strand on Sunday.
A second person has been arrested in connection with an alleged assault on a man in his 60s in Dublin.
The victim was discovered unconscious on Fairview Strand on Sunday night and was taken to the Mater Hospital with injuries described as serious.
Gardai arrested a man in his 20s at the scene.
A second man, also in his 20s, was arrested today and he's currently being questioned at Clontarf Garda Station.