Griffiths list Alexander McKinley as the only McKinley in Drumlin and he had a Sarah born or baptized in Sept 8, 1846. His wife is listed as Isabelle on familysearch and Isabelle Boyle if you check for son Alexander,1849, 1850, 1852. Son James McKinley was baptized at age 3 in 1852, John 1857, Isabelle 1844.
This would put Sarah as about 20 when she married which is reasonable.