Dozens of crannogs, the artificial island settlements built on timber and brush that Irish communities used from the Bronze Age well into the medieval period, emerged from the receding water. Among the features that came to light was a structure designated Raftery No. 19, a large cairn or possible settlement platform more than ten metres in diameter, half embedded in the south-eastern bank of the newly cut river channel. Near it, investigators recovered two stone axes, two Bann flakes (a type of carefully shaped flint tool associated with Mesolithic activity in Ireland), and a flint arrowhead, suggesting the area had been visited or occupied across a considerable stretch of prehistoric time.

The story of how this site came to be recorded is almost as interesting as the site itself. In the autumn of 1952, Dr J. Raftery and Miss E. Prendergast of the National Museum of Ireland surveyed roughly 150 crannogs that had appeared in the lakes and along the Boyle River, with Raftery allowing the project engineer, Mr R. E. Cross, to publish a numbered map of the survey in 1953. When further drainage work the following year dropped the water level again, still more features appeared, including cairns on both sides of the river below the outlet from Upper Lough Gara. Raftery was already occupied excavating crannogs at Rathtinaun and Tivannagh, but he found time to supervise a separate instrument survey of these newly exposed features. That second survey is known as the Forsyth survey, named after one of the surveyors whose name appears on the finished maps, and it is now held in the National Museum of Ireland. The two surveys do not always agree where they overlap, partly because the Forsyth work was carried out under pressure, the newly revealed features were not expected to survive, and their nature was not yet well understood. Raftery No. 19 sits at a recorded altitude of 215.75 feet above the datum of Poolbeg Lighthouse, a detail that quietly connects this prehistoric lakeshore to the tidal benchmarks of Dublin Bay.