"A parish in the Union of Conway, partly in the hundred of Creuddyn, county of Carnarvon, but chiefly in the hundred of Isdulas, county of Denbigh, on the shore of the Irish sea, 5 miles (NE) from Aberconway, containing 1133 inhabitants. The village of Llandrillo is composed of three houses only. ..... There are four weirs along the shore, where an immense quantity of fish of various kinds is taken dring the season, particularly mackerel and salmon. ..... A considerable quantity of limestone is shipped hence to Liverpool." [A Topographical Dictionary of Wales, 1833 & 1849, Samuel Lewis]
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.
The main settlements were the villages of Colwyn (now known as Old Colwyn) and Mochdre, in the townships of those names. Practically all of the township of Rhiw belonged to the Pwllychrochan estate; and, following the sale of the estate in 1865, it formed the nucleus of the present town of Colwyn Bay. Similarly, a large part of the township of Dinerth, belonging to the Bostock estate, formed the nucleus of the town of Rhos on Sea.
With the growth of the holiday trade during the Victorian period, and the increasing population, new churches were built; and eventually the parish was divided. The parish of Colwyn was formed in 1844, Colwyn Bay in 1893, and Brynymaen in 1900. Most of the township of Cilgwyn was transferred to the parish of Llanelian.
The present ecclesiastical parish is therefore very much reduced in size.
The modern town is also known as Rhos-on-Sea.