Darin- there are several Presbyterian Churches in Aghadowey parish and area: Aghadowey, Crossgar, Killaig, Ringsend. (I'm assuming your Thomas Witherow was like most Witherows Presbyterian). St. Guaire's Church of Ireland is in centre of parish.
Civil registration started in Ireland in 1864 (Protestant marriages in 1845) so there will be no birth certificate. Aghadowey baptisms start 1855, Crossgar 1839, Killaig 1805, Ringsend 1871. Therefore probably no baptismal record.
1740 Protestant Householders List has a William Witherow in Aghadowey Parish (townlands not listed). No Witherows appear in the 1831 Census (head of household only listed by name). It was certainly never a common name in Aghadowey although a few Witherows/Woodrows lived near Garvagh.
Alexander Baird of Ballynacally, Aghadowey Parish, was married to a Nancy Witherow but she was originally from Aughlish (many Witherows there) and Rev. Thomas Witherow (1824-1890) was her nephew. Thomas studied in Aghadowey under Rev. James Bryce of Killaig Presbyterian Church and stayed with the Bairds.
(this is the Thomas Witherow Christopher mentioned born at his grandfather Martin's house at Ballycastle in Aghanloo Parish).