It came, I'm afraid, almost entirely from political motivation. The biggest party in the country, Fianna Fáil, which had done pretty well and usually won a majority, saw its future as being a bit less rosy than its past had been and thought that in order to guarantee winning a majority in future elections, they would do better under a system of first past the post. It was an almost entirely politically motivated referendum on both sides.
On July 26th, 2016. See this statement in context.