I"m not 100% sure, but I think she was the daughter of Richard Mylroie & Isabel Moore. Their daughter Isabel was baptised 1721 in Lonan. If I"m right, then she was a Myroie not a Mylrea (common problem untangling those two strands!).
The couple you're referring to had 3 children I"m aware of - William, Alice & Margaret which isn't too promising so far as Isabel being the daughter of Richard & Isabel.

Wish I could be more helpful but Isabel is one of those "floating" Mylreas/Mylroies that I can't definitely pin to a family.
Do you have any more on her?