'An Post Savings Certificates' are a 5 year 6 months term deposit product with an interest rate of 3.53% AER with no DIRT. 'An Post Child Plus Savings' are a 5 year term deposit product with an interest rate of 3.37% AER with no DIRT. 'An Post Savings Certificates' offer the best buy rate for a long term, term deposit rate.
However, personally, I would not lock for 5 year 6 months, ECB rates are likely to go up over the coming years, and term deposit rates should go up as a consequence. I know you can get out of the 'An Post Savings Certificates' before maturity date at a reduced interest rate, but it might be worth considering a 1 year term deposit (Investec or Anglo) or 2 year term deposit (INBS) and then reviewing your options.