Pennsylvania Families

Her death record gives her exact birth date as Aug 29, 1831. Her 1900 census said she was born in Aug 1831. Her other censuses all said about 1835.
Her records do not record her county of birth, but just say "Ireland". But her brother James and Thomas' death records both say they were born in Co. Sligo. James was older than Margaret and Thomas was younger, therefore Margaret was probably born in Sligo also. At least that is where I would look first.

1860; Census Place: Johnstown, Cambria, Pennsylvania; Roll: M653_1088; Page: 721; Image: 727; Family History Library Film: 805088.

David McClelland, 30, laborer, born in Ireland
Margaret McClelland, 25, born in Ireland
Thomas McClelland, 1, born in Pennsylvania

"United States Census, 1860," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MXPV-5RV : accessed 14 Jul 2014), Thomas Mc Clelland in household of David Mc Clelland, Fourth Ward Johnstown, Cambria, Pennsylvania, United States; citing "1860 U.S. Federal Census - Population," Fold3.com; p. 129, household ID 990, NARA microfilm publication M653; FHL microfilm 805088.

1870 > PENNSYLVANIA > CAMBRIA > 5-WD JOHNSTOWN
Series: M593 Roll: 1317 Page: 250

D McClellan, 45, laborer,
Mary McClellan, 36,
Thomas McClellan, 11
Rebecca McClellan, 10
M. J. McClellan, female, 8
Anna McClellan, 6
B McClellan, female, 4?
D McClellan, male, 1?

1880; Census Place: Fairfield, Westmoreland, Pennsylvania; Roll: 1204; Family History Film: 1255204; Page: 178A; Enumeration District: 116.

David Mcclelland 53, farmer, born in Ireland, parents also
Margret Mcclelland 45, born in Ireland, parents also
Thomas Mcclelland 21, laborer
Rebeca Mcclelland 18
Jennie Mcclelland 16
Annie Mcclelland 14
Belle Mcclelland 12
David Mcclelland 10
Lizzie Mcclelland 8

all children born in Pennsylvania

1900 Census Fairfield Township Westmoreland Co, PA
ED 81, sheet 17B, family 343
June 19, 1900
owns a farm

David McClelland, 70, born unknown month, 1830, married 42 years, born in Ireland, parents also, arrived 1849, naturalized, farmer
Margaret McClelland, 68, born Aug 1831, 7 children, 5 alive, born in Ireland, parents also
Belle McClelland, 30, Mar 1870, single, at home
Elizabeth McClelland, 25, Nov 1874, single, seamstress

"United States Census, 1900," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/M34Y-XWG : accessed 18 Jul 2014), David Mcclelland, Fairfield Township Bolivar borough, Westmoreland, Pennsylvania, United States; citing sheet 17B, family 343, NARA microfilm publication T623, FHL microfilm 1241497.

Pennsylvania, Death Certificates, 1906-1944, ancestry.com
Mrs Margaret Hammond McClelland
died Nov 27, 1908, Fairfield Township, Westmoreland County, PA
born August 29, 1831, in Ireland
age 77y 2m 28d
white, married, female
father: Michael Hammond, born in Ireland
mother: Ann Roberts, born in Ireland
cause of death: pneumonia
buried: U.P. Cemetery, Fort Palmer, Nov 30, 1908
informant: Belle McClelland, Bolivar, RFD 1