I can't say I've noticed this trend. Not among single people and especially not among those practising monogamy. And I can only really comment about the past 10years or so, I would leave it to someone more senior than I to comment beyond that.
I'm in my mid-thirties but because of work I know a fair few twenty somethings socially, and they are no different to how we were "back in the day".
I think that alot of people in Ireland value monogamy for many reasons. In my opinion, people like security and that's inherent with monogamy. I do think that if children are involved, monogamy is good practice.
Being based in England, I can't comment on contemporary Irish sexual mores, but again, no great trend to report from this side of the Irish Sea.
If a couple does allow someone else into the relationship, even occasionally, are they really practising monogamy?