This is an excellent way of looking at these 'islands', Tis. Here's a quote from Ms Rees that might contribute to the debate
Over the centuries the surrounding land has been eroded creating an island which is now a rocky outcrop in Caernarvon Bay ... In 1802 John Skinner wrote ‘Llangwyfan Church is erected on a rocky peninsula jutting out into the sea and is an island at high water so that not infrequently the congregation were interrupted in their devotions by the rapid approach of the waves.’
The island is less than an acre ... connected to the mainland at low tide by a causeway that is two hundred metres long. The single-chambered church dates from the fourteenth century to the sixteenth century and contains some Norman stonework.
I don't know whether Llangwyfan is a tidal island now (the definitive book on tidal islands
No Boat Required only covers the British mainland, but here's some standard material (my underlinings which I'll comment on):
The most recognizable church on Anglesey, St. Cwyfan's, is popularly know as the Church in the Sea (or eglwys bach y mor in Welsh). Perched on a tiny island called Cribinau, encircled by a sea wall, this simple medieval church dates to the 12th century. It is thought to be dedicated to the Irish St. Kevin, who founded the monastery across the sea at Glendalough in Co Wicklow, Ireland.
In other words it is presumably the British end of an Irish trade route.
It may seem an odd and perilous place to build a church, but it originally stood at the end of a peninsula between two bays, Porth Cwyfan and Porth China, as shown on John Speed's map of Anglesey from 1636 (shown left).
As we have seen so often, for some reason 'Megalithic' churches are built in the most unlikely places if they were churches.
In the decades after this the sea slowly eroded the coast in the two bays enough that the peninsula was cut off, turning it into an island.
Well, maybe. But this emphasises the point I made in my Glastonbury lecture, that tidal islands don't last very long (if they are natural features).
A causeway was built to the island to allow parishioners to get to the island. (3) Its remains are visible in the picture below. However, even with the causeway, sometimes high tides prevented access. At those times services were held in a room in the nearby house, Plas Llangwyfan, which was specially consecrated for the purpose.
It seems to me baffling that people would go to the huge engineering trouble of building a causeway rather than just building a new church on the mainland.
The waves continued to eat away at the island until, in the late 19th century, some of the graves surrounding the church began to fall into the sea. At this time the church was also disused and roofless, having been replaced by a new church further inland. However, in 1893 local architect Harold Hughes, concerned for the fate of this evocative old church, raised money to save the it by constructing a seawall around the island and restoring the building
Quite so. And yet even now people still think it worth stumping up really considerable sums of money to save ... well, what exactly? The 1890's were not a time when conservation and heritage for their own sake were the rage they are now.
Although the church was initially built in the 12th century, only a small portion of the south wall dates from this period. Most of the walls were rebuilt during a 14th century reconstruction. In the early 16th century an aisle was added to the north side, accessed through an arcade of three arches, but it was demolished in the early 19th century as the cliff edge eroded ever closer. The infilled arches can now be seen in the outer wall, after the old cement mortar was removed during refurbishment in 2006. This refurbishment also involved limewashing the walls, making them very white, to the consternation of some locals who were used to the old grey appearance.
Presumably they used to be just this way when it was a navigational (or whatever) feature.