The simplest approach is to put the money into State Savings 5 year Savings Certificate.
The approach that might earn more interest is to drip feed the money into a regular saver account. A KBC Extra Regular Saver product allows a 10k lump sum and then requires deposits of 100 euro to 1k per month. The rate is 3.00% AER variable, a lot more than the State Savings product. One needs to be deposit at least 2,500 euro per month into a KBC Extra Current Account to get this rate and not pay current account fees. Also, the rate would need to be watched carefully. Too much hassle for most people but it is the strategy that is likely to earn a better return.