Hello, Seren.
First, I had to search a bit to find your first word; all four of the words you asked about have a "fada" mark over one of the vowels, which governs the pronunciation.
Hóra is apparently used as an interjection to say "Hi!" but it seems quite uncommon in my experience with the language. In fact, I have never seen it before now, but I did find it on the teangleann.ie database. I have seen some people use "Haigh" which is more of a transliteration of the English. The Irish have a different idiom for "Hi" or "Hello" which has a definitive religious slant to it, so some object to using it.
Lá breá - roughly pronounced "Law braw" means "beautiful day" but again I have not seen that used independent of some verbal structure. The Irish "roll" their "R" and there is a slight "hesitation" before the "aw" sound in "breá" that sounds like short "e" sound is inserted there, which it is, essentially; that pronunciation is used in Connacht and Munster dialects. In Ulster, it sounds more like "Brat" without the "t" at the end (and the "R" is rolled...).
Slán - pronounced "slawn" - is used to say "goodbye".