for the time being, ignore the 'which..' part of the sentence - which dates from the eighth century

The sentence will look like this:
Archaeologists in Ireland believe that a recently discovered chalice was probably buried to keep from being stolen by invaders.

without the pronoun (it) the sentence looks incomplete. We need to know what (a noun) is being buried. And hence we require IT.

Archaeologists in Ireland believe that a recently discovered chalice was probably buried to keep IT from being stolen by invaders. (it clearly has only one antecedent - chalice)

Hope the explanation helps.