It DOES matter because Patrick Kavanagh was a serious poet who would have crafted every word. He does rhyme fairly meticulously and I would be surprised if it were "play" rather than "pledge". I sing the song myself after taking a long time to understand it and I use "pledge".

I had a look on the internet and came across both versions as well as another less plausible version. There does not seem to be a definitive answer easily available.