I'm even more puzzled than Christopher. History of Congregations doesn't list a Rev. Morrison in 1st Armagh. From 1800-1858 the ministers are: Thomas Cuming (1796-1816), Samuel Eccles (1817-18223), Pooley Shuldam Henry (1826-1846), Alexander Fleming (1846-1851), John Hall (1852-1858). John Hall left 1st Armagh to go to Mary's Abbey, Dublin.
Now back to the mysterious Rev. Morrison. The Suppliment to History of Congregations lists him under Dublin, Union Chapel: "Dr. Stuart resigned in 1841; he was succeeded in 1844 by Rev. S.G. Morrison, the minister of the Independent Chapel in Plunket St. He and his congregation all came to Union Chapel. He attracted a large congregation wit an average attendence of 700. In 1861, the congregtion had 100 families ad 200 communicant members. Morrison retired in 1881. He was succeeded by the Rev. Dr. William Tait who was called from 2nd Castleblayney. He married the daughter of his redecessor. He reigned in 1890." In 1828 on Lower Abbey Street "a larger church was built and called Union Chapel, the title marking the union of the two congregations and also the union of Anti-Burgher and Burgher Synods to form the Secession Synod. Union Chapel burned down in 1916, the congregation united with Abbey Church.