Hi Betty,
Are you aware that there is a tree on ancestry for this family? The tree contains the scanned images of Sarah"s birth and marriage, Margaret (peggy) was a witness to her daughter's marriage, so no need to purchase the credits for the birth and marriage if you have a sub to ancestry
The occupation of her father John was a labourer
Sarah , married Daniel Edward Kennedy
quote 1901 census - " Margaret Jane (Peggy) Blair lived in Stranocum, Antrim, Northern Ireland, on March 31, 1901"
I still can't seem to find her in 1901 though. I put in Stranocum, left the Christian and surname fields blank and went through the names. I couldn't see anyone who looked like her - The tree owner hasn't provided a image, or a ref for this 1901 entry ,perhaps someone else can find her.
The tree has quite a few details on this family, such as a brother for Peggy
http://person.ancestry.co.uk/tree/81413182/person/34528656404/facts ( link to the tree for ancestry subscribers - you'll need to be logged in)
Betty, if it's not your tree , perhaps getting in contact with the owner may prove fruitful
Kind Regards
ps
edited to add
1901 I just found this one under Margaret Jane Blair , residents of a house 4 in Craigatempin (Stranocum, Antrim)
http://www.census.nationalarchives.ie/pages/1901/Antrim/Stranocum/Craigatempin/942934/ , living with her sister, it could be the 1901 census that the owner of the tree is referring to (?), although this census says Presbyterian. Again, this one says she is not married. Nothing under Peggy that I can find in 1901, or McAllister
I wonder if the father's name was made up for appearances sake, or perhaps she never legally married the father.
The death in 1919 in the indexes seems to be under Blair too.
Something is weird , I'm getting confused searching on her.
On her daughter's marriage cert (witness) she uses the name Margaret Jane McAllister (unless this is not her and it's another family member), and on Sarah's birth it's Peggy Jane McAllister -but after that( on the censuses ) it's Blair her former name and the censuses say unmarried . Also the death in 1919 is Blair- Are you sure the censuses and death is her? Unless she divorced, but that is rare at that time period.
Kind Regards