I am indeed Welsh, the full and proper translation which I didn't post since you have already done so Quizmonster is: The church of St. Mary, in the valley of the white hazel, near the rapid whirlpool, nearby the red cave of the church of St. Tysilio. Tysilio was a 6th/7th Century Prince who ran off to become a monk, he set up a monestry on an island in the Menai straits (which separate Anglesey from mainland Wales) which I guess the place name refers to, but later moved to Brittany where he set up a second monestry. (I seem to remember he ran off to France because his sister in law wanted to marry him and remain Queen of Powys following his brother's death!... and they say soap operas are a moderned thing!)