Evaluation of sources is a crucial part of good genealogy, so yours is an important question.

The Irish census of 1901 differs from the censuses of other parts of the UK in that year in that the original householders' schedules were preserved. So if the age on the schedule in question reads 28 then there it is reasonable to suppose that the person who completed the schedule believed your great grandmother to be that age. How accurate the age can be expected to be depends on who that person was. If it was a parent, and if that parent signed the schedule in a reasonably competent hand, then you can be pretty sure it was right. If it was a spouse, there is always the possibility that your ancestor had availed herself of a lady's privilege and shaved a few years off her age (though 15 would be extreme!). If it was an employer, he/she may have guessed rather than be bothered to ask (though again, 15 years is stretching it a bit).

Ages at death are notoriously unreliable in general and particularly so if a person died without close family of the same generation. But once again, 15 years is a very large discrepancy and a gravestone (which would have cost good money to have made) is not somewhere one would expect significant mistakes to be made.

Are you 100% certain you have the right census entry?