Thieves broke into the crypt at St. Michan's Church in Dublin and destroyed the funeral home before they flunked their heads, the Church of Ireland said.
The mummy is considered a soldier who fought in the crusades. a series of eight expeditions approved by the church between the 11th and 13th centuries, leading to a religious war between Christians and Muslims.
Intruders turned the body over while taking their heads off, and also desecrated the 400-year-old remains of a nun, according to the church.
The vandalism investigated by the police was discovered on Monday when a leader was preparing for the opening of the church.
"I am shocked that someone would attack this ancient burial ground and desecrate the remains of those in it," said Archbishop of Dublin Diarmuid Martin in a statement.
"Not only did these individuals desecrate the sacred crypt, they also destroyed these ancient mummies that have survived for hundreds of years in St. Michan ," he added.
"I would appeal to those responsible to examine their conscience and return the head of the Crusader to its rightful place."
Most of the buildings of today's St. Michan date from the 1
The crypt was previously destroyed in 1996.
"The actions of these people are devastating and sacrilegious, and we are angry at the amount of damage that has been done," said Archdeacon David Pierpoint, Vicar of the parish of Christ Church Cathedral, of which the Church is a member.
"I would appeal to anyone contacting the Gardaí (Irish Police)," he added.