Allt yr Ynys or Trewyn Mill is a corn mill that is thought to have started up as a fulling-mill. The extant building at the
Honddu / Monnow confluence has no machinery but was standing by 1754. A half mile-long watercourse off the Honddu drove a presumed undershot waterwheel.

Claire Parry, RCAHMW, 29 July 2011.

Source: A Guide to the Industrial Archaeology of South East Wales, AIA, 2003