"The church of St Michael gave the Parish its name – the Church of St Michael on the Hill. It is probably a Celtic foundation, said to have been rebuilt in the year 717AD.
The Celtic Saint Llawddog was active in this area in the 6th Century and it is possible that he was responsible for establishing a llan or small monastic settlement overlooking the Teifi. The church was built on a site that was already recognised as special: the dedication to St Michael denotes a place of pre-Christian religious use."
Steve Dubé, This Small Corner: A History of Pencader and District, Carmarthen, 2000.
The pre-Christian origins of the site of St Michael's Church is supported by the dedication to St Michael, and the Ulcagnus stone and Celtic stone found in and near the graveyard of the church. (Read more about the Inscribed Stones here.) The Church itself dates to possibly the 13th century, with extensions added in the 19th century.
As well as the possible influence of Llawddog in the area and on the Church, Vortigern (Gwrtheyrn), a tyrannous 5th century king, was also influential in the area, which is evident from the name of ‘Craig Gwrtheyrn,’ an Iron Age hillfort 1 mile west of Llanfihangel-ar-arth.
The current building of St Michael's Church is a Grade II* listed building, due to its medieval origins and post-medieval additions. There is also an ancient Yew tree on the site of the Church, which is itself a natural monument.
St Michael’s Church and Llanfihangel-ar-arth also have a fascinating modern history, from the Rebecca Riots in 1843 to the case of Sarah Jacob, ‘The Welsh Fasting Girl,’ (1857-1869).
There is more of Llanfihangel-ar-arth's modern history on People's Collection Wales.
Example of old spelling of Llanfihangel-ar-arth, as Llanvihangell Yorwoth from the will of Evan Griffith Jenkin, 1669. There are many examples of different spellings in wills and deeds on the NLW website: http://hdl.handle.net/10107/681543
History of St Michael's Leaflet