He was a friar of the Franciscan convent of Limerick and was elected Archbishop of Tuam, though never officially installed.
He was first mentioned in a letter of 1279 from Nicol Mac Máel Ísu, Archbishop of Armagh, to Edward I of England, Lord of Ireland, asking that Brother Malachy be appointed to Tuam.
Malachy may also be the author of a treatise, De veneno, on the seven deadly sins, published in Paris in 1518 and alternatively attributed to Robert Grosseteste.
The edition stated that he was a Franciscan preacher who was alive in 1300, "a doctor of theology, a strenuous expounder of the scriptures and a most zealous rebuker of vices."
John Bale recorded that he was well received in Ireland, esteemed at Oxford, and preached before Edward II.