[Kudos]

The H on the end confuses people because it's usually not really a H, it's a modernisation of the séimhiú.

I can't find any decent articles on it, but basically it used to be represented by a dot over the letter it was affecting. Unfortunately I'm not good on the recent history of the language, but the change was made some time in the last 40 years (between my parents attending school and me attending school).

The reason we have so many vowels is to match broad vowels (A, O, and U) and slender vowels (I and E) across consonants. I'm fairly sure this was a change introduced in recent history too.