How would it be spelled when I would talk about just one shoe?
The shoe = Na bróga
The shoeS = Na bróga
The singular is bróg. It's a feminine noun, so with the definite article it is an bhróg.
When specifically talking about one shoe of a particular pair (i.e. rather than one shoe of no particular pair), another possibility is an leathbhróg (literally “the half-shoe”).