I think “bansagart” is correct in itself… Depends on what you have in mind with “priestess”. If you want to refer to a “female priest”, as a female counterpart of the modern-day “priest”, then “bansagart” seems quite appropriate. However, if you’re talking about “olden times”, perhaps “bandraoi” (“female druid”) would be better? The focal.ie web site also mentions “banfháidh” as an equivalent of “female seer/wise woman”.
Go raibh maith agat, a WFM. I think I’m more referring to the older term. I’m not necessarily meaning a priestess from mythology or ancient history, but it fits much better than just the female counterpart to a modern priest. I will use “bandraoi”.