I'm not sure London to Dublin to live in digs at 16 is that different from Killorglin or Claremorris to Dublin to live in digs at 17 as people with whom I was at university did.
There's a huge difference and you're not thinking it through.
One of my best friends left home at 17 and lived in "digs" while doing A-levels. When I think back on eBay she got up to, wow! Think drink, drugs and lots of sex.
Her (Irish) parents had no idea! She hid it from them well. Because she was the only one living independently at that age, her place became the party house. I remember her confiding in me that she used to revise by candlelight, keeping the light off so friends would think she wasn't in else she'd get nothing done. That's not to say she managed well, I believe being the only one of her peers living alone probably did impact her grades.
A totally different kettle of fish to us all going off to uni at the same age.
Teenagers mature fast and there's a lot of difference between 16 and 18.
The differences between the systems have been clearly explained here, but you're not factoring in the upheaval in your DDs moving to live alone.
You're comparing the two systems as if your DD will be on an equal playing field with either. But they won't. Regardless of which country they're in or how much they feel it's their cultural home, they will be at a significant disadvantage to other 16 - 18 year olds if they're doing their leaving cert having been uprooted from their friends and immediate family and having to start again while living without close parental support.
This is a huge spanner in the works and I'm not sure why you can't see it? It's got nothing to do with people not being familiar with the Irish system and everything to do with being familiar with teenagers and human emotions!
I really feel for your DDs. What does your DP say about this? Does she mention their emotional well-being when this idea is discussed?