Do I need a VISA or Passport? Where do I get them?
Depending on your country of origin the answer can be yes or no.

If you hold a valid European passport then the answer is no. But you *do* need to make sure you bring some sort of identification with a photograph. Your passport is usually the best form of ID.
As a non-EU national (if you're a resident of a country outside of the European Union) then the answer is a resounding yes! Other forms of identification are also recommended, such as your drivers license if you intend to hire a car in Ireland.
Exceptions will include those holding a dual-citizenship. In those cases you will hold both a passport for your country of origin and an EU passport.

And what about inoculations?
Ireland does not require that you have any type of inoculations to enter the country. However, your country may require them to re-enter. Check with your local Department of Health.