An acronym is a pronounceable name made up of a series of initial letters or parts of words; for example, UNESCO for the United Nations Educational, Scientific, and Cultural Organization.
An initialism, on the other hand, is simply a set of initials representing a name, organization, or other entity, with each letter pronounced separately, as FBI for Federal Bureau of Investigation.
Form the plural of most acronyms and initialisms simply by adding “-s.” Thus, one IRA, two IRAs.
When using a or an before an acronym or initialism, use a if the acronym or initialism, when spoken, begins with a consonant sound, as in a UFO. But use an if it, when spoken, begins with a vowel sound, as in an FDA hearing.
Though the New York Times does otherwise, you should spell acronyms and initialisms without periods. Thus: FBI. Not: F.B.I.