Holy Child is a great school, and doesn't have a list of feeder schools iirc.
If you're interested, best to contact the school office directly and arrange either a private look at the school or find out the next open day.
You may end up on a wait list, however.
Take a look at Loreto Dalkey too - another really good school, and also right on the Dart. There are no places currently open for admissions in 2020 or 2021 but you can try your luck for admissions in 2022 and onward.
Also take a good look at Loreto Stephen's Green (fee paying) and Muckross Park, Donnybrook (not on the Dart but very good, very big school, non fee-paying).
For both, ask about wait lists, feeder schools. Loreto likes to admit pupils from its junior school.
www.loretothegreen.ie/wp-content/uploads/2019/03/Admissions-Policy-March-19-1.pdf
Loreto on the Green wants a letter of interest submitted two years before the expected date of admission. There is a numbered wait list and a system of prioritisation of applicants in which the students on the numbered list come last.
You apply three years ahead of expected date of entry for Muckross Park. Number 2 on their priority list for admissions are girls who are students at St. Mary's (parish) national school, Donnybrook.
You may need to apply for a large number of schools. Don't forget St Kilian's German School (co-ed) in Clonskeagh, or Newpark Comprehensive (co-ed, Blackrock).
The question is not, 'Where should we apply?' but, 'Where will we get in after applying to two dozen schools?'
Question re the sea in Killiney and Dalkey, can you go for walks along the sea there or is it all built up? I know we drove through one place that had a lovely pier/sea walk but I can't remember which town it was ( we went for a day to look around but they all blur into one!!)
That would be DunLaoghaire.
You can do the Killiney Hill walk, and swim from Seapoint south. When the tide is out, walking on Killiney Beach is very enjoyable. Take the Dart south to Bray and stretch your legs and rearrange your hair nicely by walking along the coast trail to Greystones.