From your message, it sounds as though you have very good options close by which makes this trickier to make the choice.
Bear in mind the £30k per annum will soon be £37.5k (VAT @ 20% plus 5% - normal minimum from my experience, though we did have one year of 11%). If you assume a 5% rise each year, from Y4 that looks like this:
Year Total Mth
4 £ 37,800 £ 3,150 (assumed 5% rise plus VAT)
5 £ 39,690 £ 3,308
6 £ 41,675 £ 3,473
7 £ 43,758 £ 3,647
8 £ 45,946 £ 3,829
9 £ 48,243 £ 4,020
10 £ 50,656 £ 4,221
11 £ 53,188 £ 4,432
12 £ 55,848 £ 4,654
13 £ 58,640 £ 4,887
Total: £ 475,444
Plus a couple of thousand minimum a year on incidentals (trips etc.)
You don't say how old you are (I'm assuming not too old), whether you plan on having more children or whether you're primarily reliant on one salary/ what the risk profile of your employment is/ future prospects.
I guess the thing I would say, is think hard about whether this is sustainable long term, esp if another child may be a possibility.
As others have said, it may be possible to take entrance exams to resolve the question one way or the other now, or another approach may be to save the amount above each month for a couple of years and look at the real life impact of that on your lifestyle/ family with a view to entering later (Y7 would be typical).
That would also give you a safety net if things did go wrong in the future to ensure you had fees to hand for a year or two if you're planning on funding from cashflow (as you seem to be).
We had a different challenge locally in that state provision isn't good post primary (state primary head actually recommended private for our eldest!) but in retrospect, I would have not sent the youngest to prep as soon as we did. It's made things hard and very reliant on me to keep earning at a sufficient level (which is looking a bit ropey now, though I would guess I'm older than you and we do still have a mortgage).