I've only heard it used to refer to the US, but apparently it can mean either, same as in English.
All of it, not just the United States, same as in English. e.g. Meiriceá Theas = South America.
Most countries require the definite article. Meiriceá is one of the five big ones (though there are others) that don't, along with Éirinn (Éire), Albain, Sasana, and Ceanada
Just to complete galaxyrocker's and lancet's explanation - the words "i" (meaning "in") and "an" (meaning "the") contract into the single word "sa" when they occur together. So "sa" translates into English as "in the".