He was with two friends, a boy and girl, at the river in Maynooth, County Kildare, on Tuesday at 16:00 local time.
They got into difficulty and the other boy raised the alarm.
They were brought to hospital in Dublin, but the 13-year-old died a short time later. The other two teenagers are in a stable condition.
A 13-year-old boy has drowned in the River Lyreen in the Republic of Ireland.