Jim,
The idea of a single or correct spelling for a surname or a place name in Ireland is very much a recent phenomenon designed to meet the needs of modern officialdom. Before that there was no consistency. Names were spelled phonetically and each variation was down to the whim of the particular person recording the information. You will often see the spelling change as the records go back. This rarely indicates a deliberate decision to alter the name, nor even a mistake. Not everyone was literate, but even when they were, exact and consistent spelling simply wasn’t something they bothered about.
In 1899, the Rev Smith reviewed the early records of Antrim 1st Presbyterian church (covering the years 1674 to c 1736). He noted: “Even the same word is not always spelled alike by the same hand. Indeed spelling with most of the recording officials (and they must have been fairly numerous) was a matter of the most sublime indifference. The name William, for instance, is spelled 3 different ways in as many lines; while Donegore, a neighbouring parish, is spelled 10 different ways; but these extend over a good number of years. Many families names are spelled phonetically, while others are given in the most round-about fashion.”
So expect spelling to vary. That was the norm. In addition to varying the actual spelling, O’ or Mac prefixes were optional and were often omitted. So yes your family might have used the Mac prefix at one time. Looking at the 1901 census of Armagh there certainly were McDermot(t)s there, though not all were born in the county. All were Catholic. All the Dermot(t)s were Church of Ireland, save for 2 Brethren.
If you are unable to go to PRONI yourself to search the Church of Ireland records, you could employ a researcher. Researchers in the PRONI area: http://sgni.net