Citizenship through Descent - not blood relative

Has anyone been through or heard of being able to gain citizenship if your Grandparent was Irish (born in Ireland) BUT with the caveat that they are not a blood relative?

My grandfather was Irish, he married my Grandmother when my Mother was very young - my mothers birth father had passed away.
I am not sure if there was any formal adoption but he was her father for pretty much her whole life, and my only Grandfather.
I am wondering if this can be appealed for me to be eligible for citizenship. (I am contacting lawyers as well but curious about anyone's first hand experience) .