Bellew subsequently moved to Rome, where he was given a role in the Spanish delegation and acted as an agent for Archbishop Blake. In December 1779, Bellew was appointed Bishop of Killala, possibly owing to an intercession by Henry Benedict Stuart. He was consecrated in Brussels on 3 April 1780. The appointment immediately met with controversy; on 28 February 1784, eighteen local priests sent a petition to Rome protesting against Bellew's appointment. Bellew's friendship with the Anglican bishop, Frederick Hervey, 4th Earl of Bristol, was viewed with much suspicion and his lack of Irish was criticised.[1]
By 1783, Bellew's position in Killala was more stable and he had become a respected figure in the locality. He was an associate of the local magnate, James Cuffe, 1st Baron Tyrawley. In the early 1790s, he oversaw the building of several new Catholic churches in County Mayo. He became concerned by the growing influence of the Catholic Committee, which he deemed to lack hierarchical oversight. Following the Battle of Killala during the Irish Rebellion of 1798, Bellew was summoned to Dublin Castle and forced to defend himself against accusations of sympathy for the Irish rebels; he was found innocent of the accusations.[3] He subsequently became a vocal supporter of the Acts of Union 1800, likely in an effort to demonstrate his loyalty. In 1809, he opposed attempts by Richard O'Reilly to appoint a new Archbishop of Tuam, judging it to be outside his purview. During debates on Catholic emancipation, Bellew opposed the proposal for a government veto on episcopal appointments.[1]
Bellew died on 16 June 1813, having been thrown from his carriage at Mullingar three days earlier while returning from Dublin. He was buried at Moyne Abbey beside his brother shortly thereafter.[1]