Mine is going to start in St. Brigid's NS in Killester which is a five minute drive from Clontarf (depends where you're going to live). Three of my nieces are already in that school and I heard not just from them but from others, that it's a very good school. They have after school activities such as Choir, various sports and Irish dancing.
As we're not from Killester I had to apply for different schools to make sure my wee one gets at least into one in the end. We were lucky though that we got our first choice.
Anyway I applied for eight schools two years ago when wee one turned three as all of them schools told me that application age is three at the earliest. All schools (Catholic or not, we had various) had very similar policies on how they pick the kids (locals, sisters/ brothers etc.). As previous poster said there is also Howth Road NS but I heard two different stories: one kid really liked it, the other one didn't get on there at all. Depends on the kid I suppose. We didn't get the place anyway.
Greenlanes Scotsgirl seems to become very popular. Most of the girls in my girl's Creche are going in September to that school.
Best Advice is: take the time, go to them schools you have in mind, look at them, talk to people, find out their policies etc. (School Policies turned out to be my top subject as every school has quite different ones and some of them, I didn't agree at all with so wouldn't send my child to a school I don't agree with)