The St. Michan church in Dublin, Ireland has an interesting history even without the mummy. The foundation of the church was built in 1095 to serve the remaining and ostracized Vikings, who were still in Ireland after the rest had been killed or kicked out by Irish forces in 1014. The church was rebuilt in 1686, but all along, as the church changed, the crypt stayed the same, slowly mummifying all who lay in it. The mummy found in the crypts is a nearly 800 year old crusader. It is believed that he was a soldier who either died in the crusades, or returned and died shortly thereafter.
There are a number of theories as to why the corpses in the basement have been preserved over time. One is that the basement contains limestone, making the basement particularly dry and therefore good for mummification. Another is that the church was built on former swamp land, and that methane gas is acting as a kind of preservative of the bodies. Other theories involve the presence of oak wood in the soil, or the building materials used in the church.
The Crusader was quite tall for the time, standing at 6 and a half feet tall his legs have been broken and folded up under him to fit him into his small coffin. His hand stretches out of the casket slightly and visitors were once encouraged to give it a shake. Today, you are still allowed to touch his hand, but only lightly on his long finger, due to the fragility of his old bones.