You don’t say what denomination your McDermott family was. Do you know? There are 120 McDermotts listed in the 1901 census of Tyrone. 118 were RC. The other 2 were Presbyterian (husband & wife). So statistically yours seem likely to have been RC.
If Andrew was born and later married in Strabane, then I’d have doubts about whether he was the same man as appears in the tithe applotment records for Lower Bodony. That’s about 25 miles from Strabane, and quite a distance for a farmer to go to meet a wife in the early 1800s. They tended to marry someone local to where they lived. Normally within walking distance.
Formil is between Omagh and Cookstown, a mile or so from Creggan. The tithe applotment records for Formal/Formil were compiled in 1830. To be listed there you had to have land and so most are farmers. Do you know what your Andrew’s occupation was? (If he wasn’t a farmer, he’s unlikely to be in the tithes).
The next set of comprehensive records is Griffiths Valuation which was compiled in 1859 for Formil. There is no McDermott household there in those records, so the Andrew in the tithes had apparently died or left by that year. There is a McDermott family farming there in 1901:
http://www.census.nationalarchives.ie/pages/1901/Tyrone/Crockanboy/Formil/1744234/
However the revaluation records show this family, who were RC, only took the farm over in 1884, replacing Michael Malara. So not necessarily related to the family in the 1830 tithes.
https://www.nidirect.gov.uk/services/searching-valuation-revision-books
I notice a James McDermott, in Butcher St, Strabane in 1824. He was a whitesmith and gunsmith:
http://www.cotyroneireland.com/trade/strabanepigots1824.html
I see an Owen McDermott in Camus parish on the 1796 Flax spinners list:
http://www.cotyroneireland.com/flax/spinningwheel_camus.html
Formil is in Lower Bodoney parish. The RC parish records start in 1865. Church of Ireland records start in 1812 and Presbyterian in mid 1850s.
Strabane is split across several parishes. Clonleigh RC (Clonleigh & Camus) RC has baptisms 1773-95 & 1836 -7; marriages 1778-81 & 1843 onwards. Urney Church of Ireland records start in 1807 for baptisms and 1838 for marriages and burials; Presbyterians start in the 1860s and RC in 1866. Leckpatrick Church of Ireland early records are lost, the Presbyterian start in 1835 and RC in 1863. Camus Church of Ireland in 1803 and 1st Strabane Presbyterian in 1828.
So the general position appears to be that no matter which of these parishes and denominations Andrew was born in, with the exception of Camus Church of Ireland, none has any relevant records for the dates he was born or married or had children baptised.
One possibility you may need to consider is that there simply are no documentary records of your McDermott family in Ireland.
2 documents you might want to look out at PRONI are: D3531/S/69 which is an 1849 map of Formil. Described as:
Volume of maps of Tievaleny, Formil, Tullynaboy, Kilkit and Lisdrumcleve, the estate of Hamilton and Andrew McMath; surveyed by Owen Hall, scale 20 Irish plantation perches to 1 inch. Reference tables list the names of tenants and the acreages. A survey of these townlands appears at the front of the volume and a sketch map of part of Lisdrumcleve and a map of Garrybane appear at the back.
Likewise they have D1939/2/2/106 which is another map of Formil dated 1856 which names all the tenants and records the acreages each held.
