Look more carefully and the geometry gives it away: a raised platform of dry earth, about 64 metres long and 37 metres wide, sitting inside a moat-like channel still fed by water from the lake some 300 metres to the north-west. This is the remains of a seventeenth-century fish-pond, and it was built not for mere utility but as part of a formal landscaped estate, complete with orchards, a walled garden, and a deer park.

The pond was developed around 1667 by Sir George Lane, who was then laying out the grounds of his fortified house and bawn at Rathcline. A bawn, to borrow the period term, is a defensive walled enclosure surrounding a house, common in Irish plantation-era architecture. Lane's fish-pond was an elaborate piece of hydraulic engineering. A manuscript plan of the estate, now held in the National Library of Ireland, shows the pond as a wide water-filled moat enclosing a tree-planted island platform, with triangular finger-ponds projecting inward on each side; these indentations were designed so that fish could be driven in and trapped. Water reached the moat via two channels cut from Lough Ree. One of these, running roughly 450 metres and noted on the old plan as 'The ould Trinche', measured nearly nine metres wide and almost two metres deep, and appears to predate Lane's works entirely. A second channel, labelled 'the Trinches intended', served likely as an overflow, carrying excess water back to the lake. Access from the house was through a gateway in the formal garden's east wall, then across a small wooden bridge over the moat. By 1682, the writer Nicholas Dowdall was describing the estate as being well improved, specifically naming the fishponds alongside the orchards, gardens, and deer park as evidence of its quality.

What survives today corresponds closely to that late seventeenth-century plan, which is itself rather remarkable. The platform, the surrounding moat, and both water channels remain visible. Two of the original semicircular fish-trapping ponds still project into the platform on its east and west sides. The footings of what appears to be a circular tower mark the spot where the garden gateway once stood. The wooden bridge has left no trace above ground, and the southern section of the moat has been filled in, but a stone-lined culvert, probably added in the eighteenth or nineteenth century, now crosses the south-western corner, suggesting the interior continued to be accessed long after the formal estate had faded.