Catriona is simply pronounced Ka-tree(o)-na, just as you'd normally pronounce
Katrina. The io is a diphthong (a single syllable beginning with one vowel and ending in another, like the English pronunciation of hay and eye and boat and cow), caused by "breaking" of the i before the nasal n (i.e. the vowel sound naturally changes as your mouth transitions from saying "ee" to "n"). "
Catriona" is a Scots attempt to represent the normal pronunciation somewhat phonetically.