Hi, not sure why I typed May 1870 because I have all the census' they were in as a married couple, and you're right, it does say she was born Aug 1871 in the 1900 census.
I believe I found her in the 1870 census in Unionville, Farmington, Hartford, CT. In 1880 I believe it's also Margaret I found in "The 2cnd voting district of Farmington", Hartford, CT.
Someone was nice enough to look up her obit. as follows:
1950: Easthampton, Feb 8 - Mrs Margaret Irving Purcell, 79, widow of Edward H Purcell, died yesterday at her home, 13 Chapman Ave, after a short illness. Born in Unionville, Conn, she was the daughter of the late Patrick and Mary (Riley) Murphy. She was married in Unionville to Mr Purcell in 1890, and came to Easthampton 50 years ago. Mr Purcell died Dec 3, 1949. She was a charter member of the Daughters of Isabella. She leaves two daughters, Helen of Easthampton and Grace of New York city; one son Donald of Easthampton, and seven grandchildren. The funeral will be held at the O'Brien funeral home Friday at 8:30, followed by a requiem high mass in Immaculate Conception Church at 9. Burial will be in St. Brigid's Cemetery.
So now I can probably even look up their marriage cert. and maybe if I am lucky her birth cert. too!