Always book in advance, the rates you will pay at the desk will usually be higher. Argus are an Irish broker who I've used a few times over the years. Like Premos I stick to the bigger names who tend to be more conscious of their reputations. Look up the operators where you're going to and look at their reviews on Google Maps or others. Given you need the car in the middle of your holiday, try find a convenient off-airport location as they will usually be cheaper (airports locations include a premium to cover the additional costs of operating there).
Pay for excess cover through a specialist like CarHireExcess, AIG, or Blue, far cheaper than the car hire prices.