If you are a first time buyer, you will need a 10% deposit. For a second and subsequent buyers you will need 20% of the value of the property. The reason the Central Bank has a deposit requirement is to stop a customer from falling into negative equity – this is where the property is worth less than the mortgage. The deposit acts as a buffer against any fall in the house price. So from a Banks viewpoint, the larger the deposit (buffer) the lower the risk and therefore the lower the price they charge you (interest rate)