The name Leixlip is a direct borrowing from the Old Norse "lax hlaup", meaning salmon leap, which tells you something about how long people have been paying attention to this particular stretch of the River Liffey. The salmon leap in question was not a natural feature but an artificial obstruction built across the river, a weir designed to concentrate fish and make them easier to catch as they moved upstream. That a constructed fishery could give its name to an entire town, and retain that name through centuries of English and Irish administration, suggests the structure was once central enough to local life to define the place entirely.

The documentary record reaches back at least to 1207, when King John granted Adam de Hereford rights to "all the salmon fishery of the salmon leap". The grant implies a going concern, not something newly established, which pushes the origins of organised fishing here further back still, likely into the pre-Norman period. More than three centuries later, in 1562, Queen Elizabeth granted William Vernon "two fishing places, called the Salmon Leap", suggesting the site remained valuable enough to be worth specifying in a royal instrument. The weir itself sat across the county boundary, with the Co. Dublin side falling within Backwestonpark townland, a reminder that the Liffey here marks not just a geographical but an administrative divide that older arrangements simply ignored in favour of where the fish were.