The former West Wing star discovered his family faux pas while tracing his roots for U.S. TV show Who Do You Think You Are? in Tui, Spain, where his father's family comes from.
He tells WENN, "The village is, like, 1,200 years old and there is a cathedral there that is a place that I have visited many times since 1969 when I made my first trip to Spain to visit my father's community.
"I only learned during this last trip with Who Do You Think You Are? that my great, great, great, great grandfather is buried in the cathedral there and that there was a very strong connection to my heritage from that man's life and family. I never knew that.
"In fact I used to walk over his grave and, you know, his burial spot in the cathedral; everybody did. You are just walking along. On the show they showed me where it was and I was made to be more aware of that and that was an astonishing thing."
Sheen's family tree study opened the new season of the popular genealogy show in America on Friday night (03Feb12).
During the programme, Sheen discovered the brothers of his mother and father had both served time in prison for their parts in civil wars - his uncle Michael was incarcerated in Kilmainham Jail in Dublin for his part in the Irish Civil War and his father's brother Matias fought against Franco's forces in the Spanish Civil War and was imprisoned in Pamplona's San Cristobal jail. The actor visited both prisons for the show.