You're right about the 'green' link. There is one other source for 'glas' in an (Irish at least) placename and that's the one pronounced 'glash' meaning a stream, but was often transliterated as 'glas' because the English couldn't tell one accent from another!

same as in Scottish Gaelic too - green or stream, glais and glas (I forget which way round is which)

I should also have said that Glassary is an area of Argyll (in which Kilmichael Glassary is) associated with Clan Scrimegour/Scrymegour, one of their septs being Scrymgeour of Glassary who had their seat at what is now known as Fincharn Castle on Loch Awe. it was previously known as Glassery when it was in the possession of the MacDougalls

Cheers
Andy S