Nath Í of Achonry

6th century) was an early Irish saint who is remembered as the founder of Achonry.

In the 17th century, John Colgan compiled a Latin Life of Cormac, published in the Acta Sanctorum Hiberniae series, which relates that Cormac left his native Munster for Connacht and arrived in the area of Leyney.

When Niall, the brother of the local chieftain Diarmait, begged him for a blessing, Cormac revealed that he was to have a son by the name of Conamel, whose descendants would include a number of illustrious saints, such as Náth Í "the priest".

In the Martyrology of Donegal (9 August), he is described as the priest (cruimthir) Nath Í of Achad Cain Conairi.

43v) contains a short Irish anecdote relating what happened when Nath Í, called Dathi the Priest, was once visited by Columba, Comgall and Cainnech after the monastic community had finished their meals.