Dont forget that you have other expenses that you can write off before you use the section 23. These include 12.5% wear and tear on furniture and carpets etc, Management fee's and letting expenses but to name but a few. If you figure given above are for interest only and taking into account the new rule of only allowing 755 of your interest bill as a deduction I still think you probadly wouldn't have to use any of the section 23.
Just to put your mind at rest, what I meant when I talked about you over paying for the section 23, is that the developer would have simply added on the price/benifit of the tax releif to the cost of a similar non section 23 apartment. In 10 years time if you haven't used any or all of the section 23 allowance you apartment will be worth more in a re-sale situation than a non section 23 apartment (for similar reasons).
As an example say you paid 200,000 for your apartment (section 23) with 90% relief available then you can write off €180,000 rental profit, which would equate to €73,800 in you pocket (assuming your paying tax at 41%). So in theory your apartment should be worth an extra €73,800 to an investor than an apartment that has no relief. I dont want to confuse you too much so hope that helps a bit. An remember that if the top rate of tax went up to 48% then this would increase the value of the write off to €86,400.