Firstly, it is difficult to overfund a DB scheme with AVCs. The Revenue maximum benefits tend to be well in excess of most DB schemes (With the possible exception of the older “gold plated” public sector scheme).
But any AVC scheme must be “attached” to the main scheme. So in the event of any overfunding, any excess which cannot be used to purchase additional benefits up to the Revenue limits must be retained by the main scheme. But with proper contribution advice any member contributing AVCs should avoid the risk of overfunding.