1) "sa" (= in the) affects the next word and dunks a "h" after the first letter. e.g. "bosca" (=a box) but "sa bhosca" (=in the box).
2). In the north, when showing closeness / proximity relating to a person, a place or a thing, a "h" is dunked after the first letter.
e.g. "bosca" (=a box) but "ar an bhosca" (= on the box) In the west and south, this is different (see Little Black Book over on the left for these versions)
You'll be pleased to know that words starting with l, n, r are the untouchables and are never affected!