Marie
I gather that the great-aunt was Annie McCallion born 1889 to Michael McCallion of Gortnamona who was married to Helen Bradley. Michael's parents were James McCallion and Nancy Barr. From the Iskaheen RC birth records, they had Catherine (b. 1861), James (b. 1863), Michael (b. 1866), Sarah (b. 1869). Denis (b. 1873) and Thomas (b. 1875).
If she was a first cousin, then Annie in Philly was either a daughter of one of Micheal's older siblings (if there were any) or the daughter of one of Nancy Barr's older siblings. I say older siblings because it sounds like Annie in Philly was substantially older than your great-aunt, say 10-15 years older. If so, she would have been born in the range 1874-1879.
Michael's older siblings were Catherine and James. The only record for Catherine is a son, Dan, born in 1886 with Hugh Doherty. James didn't have any children by the name Ann and Denis's children were born in Boston in 1899 or after. There is a Sarah or Sally McCallion who married James McCaul from Gortin/Ture. Their children were born about the right time but none of them were named Ann.
It appears that Nancy Barr had sisters Mary and Sarah. Mary was married to Charles McCann and they had the following children: Mary, Sarah, Anne, John, Bridget, Elizabeth, Thomas and Andrew. Anne McCann was born in 1881 which seems a little late but possible. I didn't know whether she emigrated to the US, but I thought it might be significant that she doesn't appear to have had any children in Muff parish.
On further research, I found that Annie McCann was living in Gortnamona with her mother according to the 1901 Irish Census but was not living there in the 1911 Irish Census. An Annie McCann was living in Philadelphia in 1910 at 911 N 63rd Street (near the intersection with Lebanon) as a servant in the household of Mary M Parke. It is stated that she emigrated to the US in 1904 which is consistent with the Irish Census. She was living in Philadelphia in 1920 as a servant with the Brady family next door at 1101 N 63rd Street. She may have moved to Washington, DC by 1930 but that's less sure. I have not found a death date for her.
It sounds to me as though Annie McCann is most likely the cousin who wrote to your great-aunt. Was there a return address on the card?
Steve C.