The toddler, from the west of the country, suffered serious injuries and had to be resuscitated by paramedics at his home on December 28.
The baby was taken by ambulance to Mayo University Hospital, Castlebar and later removed to Our Lady’s Children’s Hospital, Crumlin where he was pronounced dead on January 1.
Gardaí say they are now investigating the “sudden and unexplained death” of a youngster.
A woman aged in her 20s was arrested yesterday as part of the investigation. It is understood the woman is the infant’s mother.
She is currently detained at Claremorris Garda Station under Section 4 of the Criminal Justice Act, 1984.