St. Malachy O'More was born in 1095 in Armagh, County Down. St. Celsus ordained Malachy at age 25. In 1123, Malachy's uncle, the lay abbot of Bangor, resigned in favor of Malachy. Bangor lay in ruins and Malachy set about to restore it. In 1125, he was chosen Bishop of Connor and Down.

The diocese had suffered greatly from the Danish invasions and many were Christian in name only. Malachy set to work teaching and proclaiming the Gospel; he inspired the people to a renewed faith and reception of the Sacraments. He established a seminary and restored churches.

In 1129, on his deathbed, Celsus named Malachy to succeed him as Archbishop of Armagh. However, Armagh had been hereditary for generations and Celsus' family installed his cousin, Murtagh as Archbishop. To avoid confrontation, Malachy refused to occupy the see. In 1132, Malachy assumed pastoral and spiritual care of the Armagh upon the direct command of the papal legate, but left the revenues and temporal care to Murtagh.

When Murtagh died in 1134 after naming Celsus' brother, Niall to succeed him, armed conflict broke out. Niall's allies invited Malachy to a conference planning to murder him. Malachy so disarmed the group with his dignity and courage that peace was restored and Malachy became undisputed Archbishop. He restored discipline and peace, then as promised, resigned. He ordained a successor as Archbishop and became bishop of Down.

In 1137, Malachy set out for Rome. On the way, he stayed at Clairvaux and became friends with St. Bernard. When he arrived in Rome, Malachy tried to resign and become a monk at Clairvaux, but Pope Innocent II refused and instead appointed him Papal Legate to Ireland.

St. Malachy returned with the first Cistercians and founded Mellifont Abbey near Drogheda on the River Mattock.  King Donnchadh O’Cearbhaill of Airghialla supplied the materials for its construction.

In 1148, the Irish Bishops sent Malachy to Rome to make a formal petition to the Pope. Malachy stopped at Clairvaux. He was stricken with a fever and died in Bernard's arms on November 2nd. Malachy was buried at Clairvaux and Bernard was later buried beside him.

Malachy was canonized by Pope Clement III in 1190, the first papal canonization of an Irish saint. The so called "Prophecies of Malachy" were supposedly revealed to Malachy during his visit to Rome and presented by him to Pope Innocent II. They are not accepted as authentic by scholars.

An Annual Feast honoring St. Malachy is held on November 3rd.