On his return to his native country he acted for some time as coadjutor to William Ó Maolalaidh, archbishop of Tuam, and afterwards, on the recommendation of Thomas, earl of Ormonde, he was appointed the successor of that prelate, by letters patent dated 17 August 1595.
In the writ of privy seal directing his appointment, it was alleged that he was very fit to communicate with the people in their mother tongue, and a very meet instrument to retain and instruct them in duty and religion; and that he had also taken pains in translating and putting to the press the Communion Book and New Testament in the Irish language, which Her Majesty greatly approved of.
Donellan was a master of the Irish language, and continued the version of the New Testament which had been commenced by John Kearney and Nicholas Walsh, bishop of Ossory, and which was completed by William O'Donnell or Daniell, who was afterwards raised to the archepiscopal see of Tuam.
Great expectations were formed of this undertaking, and it was confidently believed that it would be the means of destroying the Roman church in Ireland.
It is a noteworthy fact that of the four scholars engaged in translating the New Testament into the Irish vernacular, three – Kearney, Walsh, and Donellan – received their education at the University of Cambridge.