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We do not have a parish liaison in Ballina that I can locate. I did find a link to the church in Ballina and you may want to e-mail the office. I assume they would know the sister. Likely, they would not be able to provide her contact info but could pass on your e-mail address. http://cashel-emly.ie/our-diocese/parishes/a-d/ballina-boher/
I tried to find your family in the 1901 census but could not locate them. Do you have the townland? I also could not find Nellie's civil birth record.