William Russell?s marriage to Catherine Elizabeth Carson was registered in Dublin South 1853 Volume 5, page 733. You can order a photocopy of that certificate from GRO Roscommon for 4 euros. Put the year, place, volume & page number clearly on the form (anywhere). http://www.groireland.ie/
Griffiths Valuation for 1857 lists just one Russell household in the parish of Lettermacaward. That was on Ranny townland. It was plot 22 which was a farmhouse, outbuildings and 20 acres held by Margaret Russell. (When a woman is the tenant it is often that she was a widow).
That farm today is near Ranny Lough, just to the east of the N56, north of Lettermacaward village.
The tithe applotment record for 1833 list just one Russell farm there then (of just over 21 acres), held by the Widow Russele, so perhaps the same lady who was there in 1857.
In the 1901 census there were two Russell households, beside each other:
By 1911 there were 4:
Though most were RC, the last was Church of Ireland.
The RC records for Lettermacaward (RC parish of Dungloe) only start in 1876, so not a lot of help for you there I am afraid. The Church of Ireland baptisms start in 1889, marriages in 1845.