If you're confident that you can choose the correct product for your needs, the correct fund(s) for your needs and the correct provider for your needs, I don't see why you can't get this done by an execution-only broker with no advice, for a lower cost than the standard.
You can set up a PRSA with a once-off contribution and add more to it later if you want. Zurich Life have an irritating system limitation that insists on generating a second policy number when you try to add a regular contribution to a lump sum PRSA, but the extra policy number doesn't cost you anything - it's just irritating to have two, needlessly. I'm not aware of other PRSA providers that do this.
There's a list of discount brokers on Askaboutmoney .