It's quite an interesting history, and it goes back a long time, nearly 100 years.
Ireland, as you know, was part of the United Kingdom, and in the U.K. there have always been electoral reformers who wanted to move to some kind of proportional representation. The model they wanted 100 years ago was STV. One of their people came to Dublin and spread the word, and that convinced the people who became the political leaders in an independent Ireland. In fact, there was surprisingly little debate. If we now look back on it, we see there was very little debate within Ireland in the 1920s, when the country became independent, as to what system should be chosen. The choice was to go for proportional representation, and people genuinely did not not know that there were lots of different types of proportional representation out there to choose from. It was assumed that proportional representation meant the single transferable vote, so that's the one we chose and that's the way it's been for the last 94 years.