The 35-year-old has since been taken to Dublin's Beaumont Hospital
A man has been arrested by gardaí investigating a serious assault in Co Monaghan on Saturday.
At around 11.30pm, a 35-year-old man was discovered on Main Street in Carrickmacross with a serious head injury.
He was removed to Our Lady of Lourdes Hospital in Drogheda and subsequently taken to Beaumont Hospital in Dublin.
His condition is described as critical.
The scene of the assault was technically examined and an incident room has been set up at Carrickmacross garda station.
A 34-year-old man was arrested on Sunday in connection with the investigation.
He is currently bring detained at Carrickmacross garda station under Section 4 of the Criminal Justice Act.
Gardaí are appealing for witnesses, or any person with information, to contact Carrickmacross garda station on 042-969-0190, the Garda Confidential Line at 1800-666-111 or any garda station.
Investigations are continuing.