Vacancy rates going up is good for renters and bad for landlords. It's not possible for it to be any other way; you simply don't make money from /not/ renting a place, compared to renting it. It's like arguing that a grocery store is hoarding food because the shelves aren't empty.

Ireland has never had an oversupply of houses in its history. A simple way to tell this is that the population of Ireland is still lower than it was in the 1800s; that's because all young Irish people move to America/Canada because they can't afford to live in Ireland.

But that's still not the best metric to look at. The right thing to look at is price; if it's too high you don't have enough units. That is the only thing you need to know. Anything else is obfustication.