I think this is one of the most beautiful names around so much so that I named my second son Niall. I have noticed it has became very popular recently. It is a common misconception that Niall is a form of Niel when in fact it is the other way round.
Has no connection to Nigel nor is it an Irish 'version'. It is an old Irish name which stands on its own. Meaning is disputed, but some think it relates to an old-Irish and middle-Irish word 'to champion' or 'successful'. There are also those who suggest it may have its roots in a word which relates to 'vapour' or 'steam' - niallach.
There is a character in Melissa Marrs books, Wicked Lovely and Ink Exchange named Niall. He was a solitary fey who worked with the Summer King until he fell in love with a human and left him. He then became the Dark King, taking Irial's place.
Don't like it. This is a name used by Irish-American parents who won't accept any non-Irish names for their kids. Please, we're not in medieval Ireland anymore!
It's pronounced Ni as in it would rhyme with eye, and finished all as in it would rhyme with ball: Ni - all . It's a very popular name in the North and South of Ireland, I once watched a documentary that claimed it is one of only 4 male names that are originally Irish as most other popular Irish Male names descend from somewhere else. Famous Nialls are Niall Quinn, famous football player and manager of Sunderland FC, and Niall McDevitt, the Southern Irish Poet. Both you can Google.
Though the NIE-al pronunciation is common in Ireland, the only legit Irish (language) pronunciation is NEE-al. NIE-al is actually an English pronunciation.