Gardaí are investigating two separate alleged assaults in Dublin city centre on Monday night.
They happened in the George's Street and Townsend Street areas of the city.
At around 2.45am Gardaí on patrol helped a man, aged in his 20s, with a serious head injury on South Great Georges Street.
He was taken to St James's Hospital for treatment - but his injuries are not thought to be life threatening.
A man aged in his late teens was arrested a short time later, and is being held under Section 4 of the Criminal Justice Act at Pearse Street Garda station.
The scene has been preserved for a technical examination.
Gardaí are appealing for witnesses or to anyone with information to contact Pearse Street Garda station on 01-666-9000 or the Garda Confidential Line at 1800-666-111.
In a seperate incident, another man in his 20s on Townsend Street was taken to hospital with minor facial injuries.
Gradaí say four men, all aged in their 20s, were arrested and have since been released without charge.
A file is being prepared for the Director of Public Prosecutions (DPP).