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I don't live in Ireland so I can't help with the grave info. I did look for civil death index records for both Michael and Mary. There were too many Michael Hynes records in the years prior to 1894 in the Gort registration district for me to make an informed choice.. There were a number of Mary Hynes records between 1901-1911 and one of them had an 1836 year of birth so possibly the record below is correct. The best way to confirm would be to check the changes in property owners after the Griffiths. Those records for the Republic of Ireland are not online yet. The Mormon church does have that information on microfilm that is held in Salt lake City.
Is it possible that Margaret was actually older than an 1873 year of birth and was baptized along with Mary in the gap period?
The Griffiths does show a John Hynes in Ballymarcahaun so possibly he was Michael's father.
Let me know if you have questions.
First name(s)MaryLast nameHynesBirth year1836Age at death73Registered year1909Registered quarter/yearApr - Jun 1909Registration districtGortVolume4Page185