Nouns beginning with vowels are immune to lenition.
Initial lenition is slightly different than lenition for plurals.
sy asar (regarding the parent)
sys asar (using the parent)
yb asar (belonging to the parent)
sy tacal (regarding the brick)
sys tacal (using the brick)
yb tacal (belonging to the brick) > yb sacal
sy mysu (regarding the cat)
sys mysu (using the cat) > sys wysu
yb mysu (belonging to the cat) > yb wysu