Fantastic detective work!
These Irish genealogy sites are new to me, so this has been a tremendous help. I think I can now be semi-confident with the following:
Maurice David Fenton (1825-1885)
married Elizabeth Rice Fenton (1831-1910) in Mitchelstown on Aug 12, 1860. They lived in Kilworth where he worked as a laborer.
Children:
1865 Mary Josephine Fenton (Lane)
1866 James Fenton
1867 Thomas Fenton
1870 Elizabeth Fenton
1872 William Fenton
1875 Margaret Fenton
1877 Bridget Fenton
1883 Maurice Fenton (probably Thomas or James' son)
1884? Adela Rice Fenton (probably Bridget)
Maurice died in 1885. His wife moved in with their son Thomas who was also hosting Mary Josephine's child. Margaret eventually joined her sisters Mary Josephine and Adela in NY.
Mary Grant Remains a mystery.
Elizabeth Rice's age is given as 70 in 1901 which would make her 46 when Bridget was born. My guess is Maurice Jr. was Thomas's son and Adella = Bridget (and the age in US records is incorrect)
Any advice on where to start looking for records relating to Maurice Fenton's parents and Elizabeth Rice's parents?
One crazy thought: What if Maurice b1825 is actually Adella's grandfather? Perhaps he had a son Maurice born between 1860-1865 who married a Mary Grant and did indeed have Adella in 1884 in Kilworth. She might have listed Margaret Fenton as a sister because of their relative closeness in age. No proof of this, just trying to solve the Mary Grant problem as the name appears on a few records. Either way, Maurice 1825 would still be related and part of the tree!