Fr Brian O Fearraigh says he has made the decision as he sees the referendum as giving statutory recognition and protection to the relationships of people regardless of their sex.
His views are at odds to that of the Bishop of Raphoe Philip Boyce who publicly stated last weekend that he was voting no.
Fr O Fearraigh has been explaining his decision further:
StandUp4Marriage, which is advocating a No vote has described Fr O Fearaigh as miss guided.
Their spokesperson is Barry Jones: