[erimir]

A long vowel doesn't have to be broken by a glottal stop. But it's indicated by a macron, so it's usually the case that multiples of the same vowel were intended to be split by an ʻokina.

For example, a type of lava flow is called ʻaʻā, with the second vowel long (the Hawaiian pronunciation would be something like uh-oh, but uh-AAH). But you may see it written as simply "aa" in English text (and "aa" is a legal word in games like Scrabble).

For other vowel sequences you can't predict whether there should be a glottal stop, however. So... Kauai is actually Kauaʻi, so the final "i" is actually pronounced separately... However, there is no [ai] diphthong, as they are separated by the glottal stop. So it's kawa-'ee

The glottal stop is a full separate consonant, and treated as a letter in Hawaiian spelling so it can appear between any vowels, or even at the beginning of a word.

The actual origin of that sound is the earlier Polynesian /k/ (so, in Maori there is a word Hawaiki, for example). While what is currently written as 'k' in Hawaiian used to be /t/ - and in fact, can still be pronounced as [t] in free variation (there's also a dialect of Hawaiian where it is usually pronounced as [t]). There are pretty clear correspondences with other Polynesian languages with those and other sounds.