Edit

Uhm, Morrighan appears in Erinn at the end of G2, inside Barri Dungeon.

Good point, did she physically appear in Barri or as a vision or narration? As I said, at some point she got unsealed, where was this?

I always assumed it was something The Miletean did near the end of G1(Wasn't that the whole point?).

Well, in G1 she was sealed in the altar room we see her in, not necessarily as her statue form in Erinn, though they might be the same.

Physically, right in front of you and Price.