I think it would be very unwise to place your 12 year old in 2nd year. Not from an academic point of view, but socially it would be very difficult for them. They will be by far the youngest in the year, probably younger than many 1st years. I have had 3 go through the secondary system here, and I really advise you against doing that, no matter what the schools might say.
A secondary school place is going to be your biggest challenge. As others have said, St Killians would be my first port of call, but most of the schools mentioned here would have long waiting lists. Colomba's in Rathfarnham has a lot of expat students, it's mixed, mostly boarding. You'd be going against traffic in the morning so the commute wouldn't be too bad. It's a nice school, good pastoral care, but at the higher end of the fees scale.
If you're having trouble finding a primary with places, Castle Park in Dalkey has a lot of expat pupils and usually has places available. Again it's a nice environment.
Don't underestimate the difficulty in finding somewhere to live! If you can access a relocation service through your employer then do, they have ways of finding houses that don't go on the letting websites.
Best of luck with your move.