So come January how many principle private residences will you have, 1, 2 or 3? Revenue will only accept you having 1. You could chance your arm with moving into the apartment for a couple of weeks and so declaring it to be your PPR but I think it's quite obvious that it's not since you may have already declared it to be an investment when you first signed the contracts (??) and Revenue aren't stupid, far from it. I would imagine it to be quite obvious too that you are effectively making a business from buying and selling property.
1. If you turn your current place from being your PPR into a rental property before you've owned it for 5 years you'll be liable for whatever stamp duty should have been paid initially for an investor on it.
2. Buying and selling the new apt within a couple of weeks will quite obviously be a business transaction and Revenue may catch you on it, thus incurring stamp duty and CGT and the profits.
Why not just flip the apartment now and delaying buying a new house till you've owned the old one over 5 years?