Same root as "glasgow", which I've heard translated as both "dear green place" and "blue-green hollows". Possibly where "glastonbury" is coming from too?
"glas: a lock, Irish, Old Irish glas: *glapsâ; English clasp.
glas: grey, Irish glas, green, pale, Early Irish glass, Welsh, Old Welsh, Breton glas, green: *glasto-, green; German glast, sheen (Bez.), root glas, to which German glass, English glass, are probably allied."
- MacBain's Etymological Irish Gaelic Dictionary
"glas: nf. g. glaise; d. glais; pl.+an, lock, fetter : glas-làmh, handcuff
glas: grey"
- MacFarlane's Scottish Gaelic Dictionary
"glass: green (of nature), verdant, soft, pale, pasty, ashen (colour), grey (of animal), raw, unfledged, sappy, callow (of youth); trigger, bond, iron; lock, stream"
- Kelly's Manx Dictionary