If you really like Niamh then go for it, sod him he's got the rest of his life to learn how to spell it!! Someone i've met through toddler group spells her daughters name "Neve" so that could be an option. We always call Niamh, Niamhy or Niamhy noo!!
I think Niamh is a fab name (I'm slightly biased!!!) although my Niamh was very nearly a "Martha"!