Hi
Welcome to Rootschat. I think it is quite likely that your g-grandfather knew my father in law's family. My fil and his brothers were all born in the early 1890s while he was at Mountjoy. We know, from the prison correspondence register which is held at the Irish Record Office in Dublin, that he went to Maryborough after 1901. By 1911 they were in Belfast and Robert was a trade warder at Crumlin Road. They remained in Belfast after retirement and from there the 3 brothers all joined the British Army. The prison cottages at both Mountjoy and Portlaiose have been demolished. The Mountjoy cottages were demolished to build the women's prison.
Records for prison staff are hard to access - we were told (although it was a few years ago now) that there is only the correspondence register accessible for research.
I am guessing that you are in the States but contact with
http://www.nationalarchives.ie/ may be helpful. Good luck with your research - Ellen