The short answer to your question is, it is possible to apply for a new passport at any time during the 10 year span. However, even if your existing passport is still current, you will still have to pay the full 75 euro fee.
For security reasons it has never been possible to download passport application forms - nor it is possible to appy or pay for a passport online.
The Passport Office have indicated these services will/may be made available in the future but to date, no country I am aware of, facilitate these services.
As previous posters mention above, you may pick up a passport form at any post office or Garda station. The An Post service referred to above, will allow you to have your new passport within 10 working days.