Pen-yr-Allt (the nearer building) was originally a farmhouse built in the mid 19th century. It was later rebuilt and served as the home of the head forester of the Plas Newydd Estate. Grade II listed - see Link.
Beyond is Glan Aber, lodge to Aber-Braint. The lodge was built in the early 19th century and is also grade II listed - see Link.
I believe this particular sign is now in the industrial railway museum at Penrhyn Castle, Bangor NH6071 [ http://en.wikipedia.org/wiki/Penrhyn_Castle_Railway_Museum ]. The translation as given to me reads 'The Church of St Mary by the hollow of white aspen, over the whirlpool and St Tysilio's church close to the red cave'.