Hi AbiGilby, it's pronounced Neev and is from Irish mythology, the story is..
In Celtic mythology, Niamh (pronounced /niːəv/ or /niːv/) was the daughter of Manannan mac Lir and Queen of Tir na nÓg. She came down on a magical horse, Embarr, one day and asked Fionn mac Cumhail if his son Oisín (pronounced /ˈɔʃiːnʲ/ or 'USHeen') would come with her to Tir na nÓg. Oisín agreed and went with her to The Land of Youth, and promised his father he would return to visit soon.
Oisín was a member of the Fianna and, though he fell in love with Niamh and returned with her to Tir na nÓg, he became homesick after what he thought was three years. Niamh let him borrow Embarr, who could run above ground, and made him promise not to get off of the horse or touch Irish soil.
The three years he spent in Tir na nÓg turned out to be 300 Irish years. When Oisín returned to Ireland, he asked where he could find Fionn mac Cumhail and the Fianna, only to find that they had been dead for hundreds of years. Whilst travelling through Ireland, Oisín was asked by some weak men to help them move a boulder. He tried to help them from his horse, but he fell, and upon touching the ground he instantly became an old man. He is then said to have dictated his story to Saint Patrick, who cared for and nursed him until he died. Meanwhile, Niamh had given birth to his daughter, Plor na mBan. Niamh returned to Ireland to search for him, but he had died.
Apparently the name itself means "Bright". We really liked the name and although neither DH or myself are Irish, DH is Scottish, so is happy to have something with a Celtic link, albeit not a Scottish one.