If it's for retirement, have you considered making larger pension contributions from your income? Get it into the fund tax free and let it grow tax free as well.
After that, if it were me I'd be doing as per qwerty5 and sticking it in the stock market. With fees etc. you may find it cheaper to buy €1500 a quarter, rather than every month. With the size of your fund it would not be advisable to buy individual stocks as you'll struggle to achieve any real diversity, so you'd be better off with the likes of ETFs where you can put say €1500 into the FTSE100 one quarter, €1500 into the S&P500 the next, €1500 into BRIC or whatever takes your fancy.