Niamh

Niamh should become Néimhe in the genitive case & should in former times have become Néimh in the dative case. [The dative case is becoming archaic in Irish today [except in cases where it has itself replaced the nominative]]. But though the genitive is much more usual in modern usage, this does not seem to apply to many personal names. An bhfaca tú leabhar Niamh? would be a more usual usage today than the more correct an bhfaca tú leabhar Néimhe? perhaps.
One explanation for this might be that the name was not in common use as a girl's name in earlier centuries and only existed in the story of Oisín mac Fhinn going to Tír na nÓg with Niamh Chinn Óir [Bright-(one) of (the) Head of Gold]
Searching all the names in the 1911 census for county Dublin there was one Niamh & one Neave. These two were both young children which suggests that the name had not been used in the 19th century.

Niamh came from Tir na nOg or the land of the ever young. She fell in love with Oisin a mortal and took him away with her on her white horse to become immortal. He missed home and went back on her horse to visit being told not to put his foot on Irish soil. He saw the Irish struggling against their enemies and he leant down to help them but the girth on his saddle broke and he became as old as he would have been. Legend says he told his story to St. Patrick when he died and swirling white clouds are Niamh coming to look for him again.

Niamh was the daughter of the King of Tir na nOg and not the daughter of the sea king. She dwellt in the land of the ever young where she still rides out on her white horse looking like a cloud in the sky.

In Irish mythology, there is the story of Niamh and Oisin. Niamh is the most beautiful woman in all of Ireland.