The ancient parish of Llandrillo yn Rhos lies in the extreme north-west corner of Denbighshire. It originally contained the townships of Eirias, Colwyn, Rhiw, Cilgwyn, Llwyd Coed, Mochdre, and Dinerth (also spelled Dinarth). It was bounded on the east by Llysfaen parish, on the west by Llangwstenin and Eglwysrhos parishes (in Caernarfonshire), and on the north by the sea.
Until 1924, the parish of Llysfaen and the adjacent township of Eirias formed a detached portion of Caernarfonshire, within Denbighshire. There was also a detached portion of Eirias township in the extreme north-west corner of the parish.
St. Trillo's church was mentioned in the Norwich Taxation of 1254. It was whitewashed, to serve as a landmark for ships at sea. The church was restored in 1857, further work being done in 1873 and 1898.
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For family history information about the churches and parish, visit the GenUKI Llandrillo yn Rhos page.