Have you considered the 1883 death in Ballymoney district of an Isabella M'Dowell [sic] born c1813? Always worth checking in case surname was mis-transcribed. Or the Isabella McDonnell death in the district in 1865 aged c15 - again, worth checking in case age mis-transcribed.
Also, on any records, check under spelling 'Gortereghy' for the townland too.
Above deaths unlikely as Isabella still in Valuation Revision books after these dates. Indeed, Isabella's name appears until c1897 then in next book commencing 1898, at map reference 13 where Isabella had previously been recorded, it is now a James McDonnell. Unusually, the book commencing 1898 does not have Isabella's name recorded and scored through with James as replacement, it only has James entered, yet seems to be same property as has map reference 13, as did Isabella's property in the previous Revision Book.