I really really wouldn't send a child to John Scottus. I've had a few friends have children there, and we considered it for ours and did try-out days. It's extremely cultish due to its association with the school of philosophy. All my friends ended up taking their children out due to increasing pressure as they moved up through the school to conform to the school of philosophy's ethos. IMO your dcs would settle better in a state national school. Class sizes are big but education is very child-centred and they would learn more about Irish culture and be better integrated in a national school.
If you're committed to Sandymount:
Shelleybanks is a nice school, it's got a close relationship w the relatively new (6 years old) Educate Together secondary in Sandymount which is still finding its feet a bit - but just so you know your dcs classmates may go on to that school so might be worth checking out. If you live in Sandymount, a lot of kids will go to secondary along the DART line so you might want to think about what schools would work for you in that case.
Sandymount is pretty but not great for cross-Dublin public transport other than the DART, and I wouldn't buy a house there due to potential future flooding (married to an engineer!) The commute to the airport might look ok on paper but could end up being a total pain.
If you'd consider moving closer to the airport:
As others have mentioned, Howth and Sutton are lovely - but places closer to the city are also nice (and very convenient) with lovely houses such as parts of Drumcondra which has excellent state primary schools.
Skerries was recently cited as the ideal place to live by an English anthropologist! https://www.theguardian.com/world/2023/dec/02/has-irish-town-found-secret-the-good-life-skerries
In terms of secondary schools in north Dublin, there's Sutton Park and Mount Temple as others have mentioned - Skerries Community College (state) is a great school. If you have boys and don't mind single sex, a lot of boys in North Dublin go to Belvedere College in the city centre which is one of the best schools in Ireland. It's Catholic but religion tends to be manifest in a commitment to social justice rather than dogma, and there are plenty of non-practising and non-Catholic students there.
Good luck with the move!