From there is continues eastwards through the village of Knockvicar to the River Shannon at Lough Drumharlow, near Carrick-on-Shannon.

The length of the Boyle River (from its source in County Mayo to the Shannon) is 64.4 km (40 mi).

[2] The Upper Shannon catchment, above Carrick-on-Shannon (area: 1,301 km2[3]), has two distinct reaches, the River Shannon (basin area: 576 km2), which rises in County Cavan, and the Boyle River (basin area: 725 km2), with its source in County Mayo, which have their confluence at Lough Eidin/Drumharlow, approximately 4.7 km upstream of Carrick-On-Shannon.

The river flow from the furthest reaches of the Boyle catchment to Limerick City has a measurement of 290 km (180ml).

[7] When added to the Shannon's 102.1 km (63.5ml) estuary, this gives a total river length of 392.1 km (243.5ml), which makes it the longest river within the Shannon River Basin (from source to mouth), 31.6 km (19.5ml) longer than the Shannon Pot source.