The River Blackwater, also called the Kells Blackwater or Leinster Blackwater, is a river that flows through the counties of Cavan and Meath in Ireland. It is a tributary of the River Boyne which is one of seventeen rivers that flow into the Irish Sea. (The River Leinster Blackwater is one of two River Blackwaters which flow into the Boyne in County Meath, the other originates in County Kildare).

It has its source in the south of Cavan, near the town of Bailieborough. It flows through Lough Ramor at Virginia, County Cavan, past Kells, County Meath, before joining the River Boyne in Navan, County Meath.

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