The river takes its name Súileach ("sharp sighted" or "seeing one"), a multi-eyed man-eating water monster that was reputedly killed by Saint Columba.

These include the Forglug Burn, which flows into the river just to the south of Conwal Cemetery, and the Correnagh Burn, which flows under the Derry Road (part of the N56), entering the river at Bonagee on the eastern edge of Letterkenny.

The river was extremely hard for larger ships to navigate, as it was narrow and has many bends near Letterkenny.

The coal yard still remains on the old site opposite the Mount Errigal Hotel.

[citation needed] The Swilly is spanned by numerous bridges mostly open to road traffic.