The parish was originally a chapelry of
Llanrhaeadr-ym-Mochnant, but was later formed into a separate parish, consisting of the
township of Tre'r Llan, where the parish church was located, and those of
Tregeiriog (the only village, in the neighbouring valley) and Nantyr, both of which were entirely detached from Tre'r Llan and surrounded by other parishes.
Samuel Lewis's 1849
Topographical Dictionary of Wales described the parish as having 234 inhabitants and covering 2900 acres in total. In the late 1980s, Tregeiriog and its surrounding areas were transferred to the parish of
Llanarmon Dyffryn Ceiriog. There was a corresponding
civil parish of Llangadwaladr, but following 1974 boundary and administrative changes, some of the area is now part of the civil
community of
Llansilin; the remaining parts of the civil parish are now in
Ceiriog Ucha in
Wrexham county borough. Before the rural depopulation of the late 19th century, the area was almost exclusively Welsh-speaking. The 1891 census recorded that 99.5% of the inhabitants of Llangadwaladr parish spoke Welsh, and that 88% knew no English whatsoever. A correspondent in
By-gones, relating to Wales and the Border Counties, stated that an elderly local had told him about a form of
"mob" or "folk" football formerly played between the parishes of Llangadwaladr and Llanarmon Dyffryn Ceiriog, where the opposing teams would "contend from morning even until night, and sometimes [...] fresh help would be fetched up in the evening". ==St Cadwaladr's church==