Bhreithlá? I is slender, right? And A is broad. How does that even work; I thought that was never supposed to happen?
When mo or do come before a vowel sound, they become m' and d'.
Note that it's "a vowel sound". Mo and do cause lenition, so f becomes fh and fh is silent, which exposes the sound of the following letter. If fh comes immediatelybefore a vowel, it counts as a "vowel sound", so you will use m' and d' instead if mo and do.
m'athair, m'eilifint, m'fhón, m'iníon, m'obair, m'úll
d'arán, d'eitleán, d'fhear, d'im, d'oifig, d'uachtarán
Note that this isn't optional - you must drop the "o" - mo athair would be incorrect.
(Edited to clarify that the silent fh exposes the following sound, which can be a vowel or a consonant, as suggested by scilling below).
With a word beginning with f, one has to look at the next letter to decide whether fh results in a vowel sound. For example, fón would use m’fhón because ó is a vowel, but freagra would use mo fhreagra because r is a consonant.
No, lá is masculine, so it's lá breithe sona duit and breithlá sona duit. (breithe is an adjective in the first phrase, and compound words like breithlá take their gender from the 2nd part)
By the way if you put an asterisk (*) at the beginning and end of your phrase, it will italicize it.