I have an early version of Discoverer map 19, which covers northwest Co. Armagh. These older map versions didn't attempt to show all of the townlands, and I can't find a Dressogagh in Tartaraghan civil parish - although one is supposed to be there. It could be near Summer Island, which I DID find in that parish. It's identified as a "Park or Ornamental Grounds". It is located about 6 or 7 miles SE of Dungannon town (and 2 miles SE of Moy), and is shown as an area about a half mile by a quarter mile in size, with a large building at the north end of the "park", along with a second, smaller building. At the southern end of the park, on the B 28 road, there are three small buildings shown next to the park entrance road, which continues northward to the main building.

Incidentally, there is also a Dressogagh townland in Eglish civil parish. This should be on this same map, number 19 - but I can't find that other Armagh Dressogagh either. Perhaps the newer editions of the NI's Discoverer map 19 will show these two townlands.