Llansillin parish has some parts in Denbighshire and some in Shropshire. Also, the inhabitants of these border parishes seem to use the closest parish. Some of my ancestors who lived in the south eastern part of Llangollen parish, on the border of Denbighshire and Shropshire, used Selattyn church.
I grew up in the area and, even in the 1950s and 60s, it was administratively a wee bit illogical. For example, some school friends of mine who lived in Llansillin were bused to Llangollen and had to stay there for the week instead of going to school in Oswestry, which was much closer.
Gadget