Why do the Gaelic words for France and Germany take a form of "the" before it, thus letting me use "sa", but America doesn't?
Most countries are definite, but five big ones aren't: America, Ireland, Scotland, England and Canada
A little background can be found in this discussion. Note that Ireland and Scotland generally require a “the” in the genitive — exceptions for this genitive article are mainly governmental uses like Dáil Éireann.
He’s only here until he gets to go back to Ireland!