The location and timing of the Mary D. Flewellin you mention suggests several possibilities. It is 99% certain that Mary was likely descended from Thomas and Hannah (Smith) Flewelling of Hempstead, Long Island, NY. The Westchester Co. setting suggests the family of Robert Flewelling (son of Thomas and Hannah) and Maplet Flewelling of North Castle (now Armonk), Westchester Co., NY. Robert and Maplet's daughter, Mary, (by Robert's will), m. Joseph Green. This would suggest that Mary D. Flewellin (this branch of the family later used Flewwellin and Flewellin) may have been Robert and Maplet's grandaughter or great granddaugter.
One son of Robert and Maplet was Robert Flewwellin, Jr. He m. Mary D. Searles. Their daughter, Mary, m. Jesse Lounsberry.
A law suit that outlines many of the descendants of John and Mary Flewwellin includes the statement:
"John Flewwellin, son of Benjamin Flewwellin, died in 1885 leaving him surviving as his only heirs at law and next of kin:
Clarissa Smith, Mary McKeel, Susan Matthews, Ophelia Quimby and Horace P. Flewwellin."
The 1850 census of New Castle (once part of North Castle) gives Mary D. (b. ca. 1845) as John's (b. ca. 1811, d. 1885) daughter. John's wife, at the time, was Eliza A. (b. ca. 1812.) John Flewwellin appears to be the s/o "Benjamin Flewwellin b. 15NOV1815, d. 26AUG1892, m. Martha Horton (b. 16AUG1817, d. 7AUG1886), d/o Henry and Jane (Quereau) Horton of Yorktown"; and John and Eliza's children appear to have been:
Horace P. Flewwellin (b. ca. 1833 - 1900)
Susan L. Flewwellin (b. ca. 1836.-.1889)
Ophelia Flewwellin (b. ca. 1839 - 1883)
Clarissa Flewwellin (b. ca. 1843)
Mary D. Flewwellin (b. ca. 1845)
Admittedly, there is an element of doubt in these statements as the dates do not fit. I suspect that John could be the s/o Robert G. and Martha (Wright) Flewwelling), rather than of Benjamin. In any case, Benjamin was also the s/o:
ROBERT G. FLEWWELLIN b. ca. 6(or 16)DEC1773, d. 22SEP(or 2OCT)1840, bur. Fair Ridge Cem., Chappaqua, NY (Nancy Tompkins Carrasco Tompkins GenForum # 1278, 4JUN2001), m. MARTHA WRIGHT (b. ca. 27MAR1776, d. 12OCT1831, bur. Fair Ridge Cem.)
Benjamin's brother was:
JOHN S. FLEWWELLIN (See comments on John [R4184]) b. 18NOV1809, d. 28AUG1885, bur. Quaker Burial Ground, Purchase, Westchester, Co., NY, m. 15DEC1830 ELIZABETH “ELISA” ANN PURDY (b. 12JUN1812, d. 9FEB1895, bur. Quaker Burial Ground, Purchase, Westchester, Co., NY)
which would appear to be the John and Eliza in 1850. I would suspect that the lawsuit made an error in wording. It thus appears that Mary D. was the d/o John S., s/o Robert G., s/o Robert, Jr. and of Mary D. Searles. Robert, Jr. was the s/o Robert and Maplet Flewelling, with Robert the s/o Thomas and Hannah (Smith) Flewelling and Thomas the s/o Thomas and Hannah (Ashman) Flewelling.
Ira ‘McKeel’ was a grocer in Purdy, Westchester Co., NY in the 1880 census. His wife, Mary D., was b. ca. 1846. Children were: Clara L. (ca. 1872), ‘Mortimor’ (ca. 1876) and Niles S. (ca. 1879.)
Interestingly, the Mekeel name, (Loyalists to New Brunswick, Canada in 1783), also went through similar permutations in NB, becoming McKeil and McKiel. (Or McKeel on occasion.) There they are connected to Mary D.'s Loyalist cousins, the Flewelling's; my part of the family. I believe the name is actually Dutch in origin, rather that Celtic.