Mrs. Sullivan was an elderly resident at the Cedars of Lawndale hospital in "Ill", and had the room before Daria. Dr. Phillips and the other stuff mutter darkly about a horrific ordeal Mrs. Sullivan went through, giving the impression she died horribly - and thus Daria has a nightmare of Sullivan's ghost coming to take her to Heaven.
As it turns out, she's alive - she'd just been having a lot of enemas. (Huh huh huh) She turns up at the end to see if she left her enema bag in the room.
In Daria's dream, she's "Eleanor Sullivan" but this is not said in the real world.