Been meaning to answer this from Rahtz's Myth and Archaeology of Glastonbury...
He takes Glastonbury to be A/S which is pretty obvious, 'byrig' which either means a strong place or a monastery, the 'Glasteing', (first used in charters 7th/8th century) the name of a person who founded an A/S estate..there is a story that a man named Glasteing came down from the north chasing a sow to Glastonbury, and finally found her suckling her piglets under a fruit tree next to the old church...a good celtic saint story, especially with the pig....
'Inyswitrin' (British or Gaelic) attributed to a 601 ad charter, but not really accepted as genuine, its not used to the 12th century, and then the word was of course interpreted as the Island of Glass = Glas..

The other thing of course is the Chalice Well which was excavated in 1961;
Original ground surface was reached at a depth of four metres and had flints and roman pottery, it was after all the principal well for the Tor. But what was interesting was that it had the stump of an old yew by the well dated to the Roman period... which seems to suggest that yews and celtic christianity gives evidence for a very early establishment here on top of a pagan shrine ??