"I like my sister."

Translation: Is maith liom mo dheirfiúr.

[artiguesmommy]

I am confusing when to use m' and when to use mo.

[SatharnPHL]

mo becomes m' comes before a vowel sound - words that start with a vowel, or with f followed by a vowel (because mo causes lenition, and fh is silent, exposing the sound of the following letter).

m'athair
m'eochair
m'fhón
m'iníon
m'oifig
m'úll

This contraction isn't optional.