The discovery in County Waterford was made shortly before midnight.
A woman in her 30s was arrested near the scene in Dunmore East and is being detained under Section 4 of the Criminal Justice Act 1984 at a nearby police station, said police.
The boy's body was removed to Waterford University Hospital for a post-mortem examination.
The scene is preserved for forensic examination and the local Coroner and Office of the State Pathologist have been notified.
Irish police said inquiries are ongoing and the results of the post-mortem examination will determine the course of the investigation.
A police spokesperson said: "Results of the post-mortem will determine the course of a Garda investigation.
"Enquiries are ongoing, and additional updates will be provided as available.”