Lewis Ferdinand is shown as Roman Catholic on the 1901 census, and his 2nd marriage to Kate Webb in 1874 also appears to be Catholic*. Unfortunately records for the Catholic Parish of Drumragh Co. Tyrone only cover back to the 1840s, so not quite far enough to include a baptism of Lewis.
for reference : 1901 Census Ferdinand Household, Eyre Square, Galway
I dont see any sign of Denis on Griffith's Valuation in Drumragh, possibly already deceased by the time this took place.
* the records states "according to the rites and cermonies of the Roman Catholic Church", and the Bishop of Galway, but also has some notation about a house [?]