Both features appear on a Longfield map held in the National Library of Ireland, and both have since vanished from the landscape entirely.

The Longfield family were prolific cartographers of Irish estates during the eighteenth and nineteenth centuries, and their surveys frequently captured functional agricultural and hydraulic infrastructure that later OS mapping or aerial photography would miss altogether. The weir at Derreenasoo appears alongside a wooden bridge, itself now gone, suggesting a small but deliberately engineered local network, the kind of modest water management that would have regulated flooding, supported mills, or maintained drainage across low-lying ground. That neither structure survived, and that the weir left no visible trace, places it among the quieter casualties of landscape change, the sort of thing that disappears not through dramatic event but through gradual disuse and the slow reclamation of the ground.