In 1896 Pope Leo XIII confirmed the immemorial cult of the "Blessed Thaddeus". The same year (1896) clerics in Ivrea agreed to donate major relics of Mac Cárthaigh to the dioceses of Cork & Ross and Cloyne. These relics can now be found in The Church of The Immaculate Conception The Lough, Cork North Cathedral, Cobh Cathedral and St Mary's Parish Church in Youghal. His feast day of 25 October is kept in the dioceses of Ross, Cork, and Cloyne, and in Ivrea, where he was buried.
