There are no birth registrations for this period so you will have to rely on family documents for the actual birth date.
Baptism usually took place within a few days of birth.

She would appear to have at least one son John born circa 1878.

1861, Lonan, district 4, schedule 56, Balladhoo
Mary Redpath, head, unmarried, 17, farm 27 acres, Ireland
Sarah Ann Garret, niece, 5, Isle of Man

1861, Patrick, district 5, schedule 57, Intack
William Garrett, head, married, 30, lead miner, Onchan
Anne, wife, married, 27, domestic affairs, County Down
John J, son, unmarried, 9, scholar, Onchan
Mary, daughter, unmarried, 4, Patrick
William, son, unmarried, 2, Patrick
Edward Alex, son, unmarried, 6 months, Patrick

1881, Patrick, district 5, schedule 84, Phoby
Ann Garrett, head, widow, 47, Ireland
Edward, son, unmarried, 21, lead miner, Patrick
Agnes, daughter, 16, Patrick
Joseph, son, 10, scholar, Patrick
John Swinburn, grand-son, 2, Liverpool