First things first - stop overpaying your mortgage.
1) Negative equity mortgages are exempt from the Central Bank's new mortgage lending limits, so, it pays to be in negative equity.
2) Bank of Ireland requires you to have 10% of the purchase price of the new property. So, if the property you want to build costs €300k, you will need your €30k + more for the transaction costs.
Bank of Ireland explains its Negative Equity package
here. It seems you qualify to build a house up to a value of €300k
Maximum LTV for Self Builds - 125% (including negative equity)
I am not sure how a new build works in practice. (Selling and buying a new house for a contractual prices is straightforward enough)
I presume you have the site?
Do you sell the house now and keep up repayments on the mortgage shortfall?
Then draw down money for the building costs as you build?
If the €300k is a bit tight, you could consider waiting another year. Your savings will go up to €60k. If house prices don't change, you will build up a bit more equity in your home through the capital repayments.
But presumably a self-build is a long project. So it might be best to put your apartment on the market now. This locks in the negative equity at the current prices. This protects you against house price falls. And, bizarrrely, it protects you against house price increases which might lose you your exemption from the new Central Bank rules.