An other possibility is that she got whatever was coming to her at the time of her marriage, but one woukd expect a mention.

In 1881 there is an Isabella Gilmour, born in German aged 87, and living with her daughter Anne, 35 (52 years younger!) in Braddan RG11/5606 p 5. This Isabella appears in the Brian's burials index on March 11 1888 aged 95. This may not be the same person, but I can find no other candidates.