On the Usher Web page in the Irish section under The USSHER Family of Cappagh, Co. Waterford there is a Mary Ussher listed, father John, and she has brothers William and John. This could be the person you are looking for. She is in the tree athttp://freepages.genealogy.rootsweb.com/~usher/USHERCAP/73.H...
Hope this is of some use to you.
The Real Houses of Usherhttp://freepages.genealogy.rootsweb.com/~usher/