Niamh - say Neev, or if you want to be really pedantic, it almost has 2 sylables - Nee- av - soft on the v, making it almost like an f, and say it quickly, not dragging the 2 sounds/syllables out.
In legend, she was actually the daughter of the Sea-God, with long blonde hair and blue eyes, and she does entice Oisin ( say o-sheen), who was a very handsome and strong warrior, to the Land Of Eternal Youth.
There they lay around all day, feasting on the best of food and making love. They had 6 children from memory. Then one day Oisin says to Niamh, "I don't know how long I have been gone, but I must return to Ireland to see my father ( whom I believe was a mortal King)."
Niamh begs him not to go, but he insists. She then gives him a white horse to ride and says " Go if you must, but do not EVER get off the horse."
He goes back to Ireland and gets off the horse. The moment his feet touch the ground, he becomes a very weak, ill old man. He has been in The Land Of Eternal Youth for over 600 years. He dies and Niamh is left to weep for him for all Eternity.
That is just off the top of my head, the story when written down completely is some pages long, but the basics there are correct.