A Mass rock was a rock used as an altar by the Catholic Church in Ireland, during the 17th and 18th centuries, as a location for secret and illegal gatherings of faithful attending the Mass offered by outlawed priests. Similar altars, known as Mass stones, were used by the similarly illegal and underground Catholic Church in Scotland, membership in which was similarly criminalised following the Scottish Reformation in the mid-16th century.

A priest hunter was a person who, acting on behalf of the English and later British government, spied on or captured Catholic priests during Penal Times. Priest hunters were effectively bounty hunters. Some were volunteers, experienced soldiers or former spies.