William Atwell’s death is registered in Dungannon Jan – Mar 1882 Volume 1, page 445. Age at death gives a year of birth of 1810.
Robert Gibson’s marriage to Mary Ann Atwell is registered in Dungannon Volume 5, page 494.
You can get a photocopy of both certificates from GRO Roscommon for 4 euro each, provided you quote all the references given above and they don’t have to do any searching. www.groireland.ie/The marriage cert will obviously tell you which church the marriage took place in.
It was traditional to marry in the wife’s church. Thereafter the family would usually attend the husband’s church. There are 2 Presbyterian churches in Dungannon. The 1sts baptism and marriage records start in 1790. They have been copied and are in PRONI, Belfast. You should be able to order a copy in to your nearest LDS library. The 2nd’s baptism and marriage records start in 1830 & 1825 respectively. They remain in local custody (ie with the church).