This name popped in my head, WDYT? I think an Arthur character had this name but I'm not sure. Any combos or nicknames come to mind? Is it useable or GP worthy?
I agree with Bear, I think you might be thinking of Fenella. I checked an Arthurian name database and didn't see Fernella. There's no Fenella, either. But I love Fenella and Fionnuala. I think either are usable, but Fenella much more so.
Are you sure you're not thinking of Fenella? It's a variant of Fionnuala, which isn't Arthurian, but has a rich Irish folk-lore history of its own. I don't recall any Fernellas, althought I could be wrong.