A fish weir, in its simplest form, is an artificial obstruction built across a river to trap or redirect fish, exploiting the natural behaviour of salmon making their upstream runs. The spot here was known as the Salmon Leap, a name that signals exactly what drew people to it across the centuries, and the fishery it supported left a paper trail stretching back more than eight hundred years.

The earliest surviving record comes from 1207, when King John granted Adam de Hereford 'all the salmon fishery of the salmon leap', a royal gift that places this particular stretch of water firmly within the orbit of medieval economic life. Fisheries were valuable assets in this period, carefully controlled and frequently traded between powerful hands. More than three and a half centuries later, in 1562, Queen Elizabeth granted William Vernon 'two fishing places, called the Salmon Leap', suggesting the site continued to be recognised as productive and worth holding in formal tenure. Both grants are noted by the fisheries historian A. E. J. Went, whose 1954 work documented Irish salmon fisheries and their documentary histories. The fishery itself comprised an artificial obstruction across the course of the river, the weir that gives the site its current designation.

The weir no longer functions as a fishery, and what remains on the ground at Backwestonpark is a matter for close inspection rather than dramatic spectacle. The site sits within the parkland of Backweston, west of Leixlip in north County Dublin, an area whose river corridor has seen considerable change over time. Visitors with an interest in early industrial or medieval water management will find the location most rewarding approached with Went's research in mind, since the physical remains are subtle and the significance lies largely in the documented continuity of use across royal administrations separated by centuries. The Salmon Leap toponym itself is worth noting as you look at the water; such place names in Ireland frequently preserved the memory of fish runs long after the structures that exploited them had disappeared.