The dash isn't needed when you prefix "h" to a vowel (because native Irish words never begin with "h" in their basic form, there is no confusion):
go hálainn, go hiontach
But when you capitalise these words, the "h" remains in lower case:
Go hÁlainn, Go hIontach
When "t" or "n" are prefixed to a vowel, on the other hand, you need a dash:
an t-éan, na n-éan
When capitalised, the dash is not used:
An tÉan, Na nÉan