A sub-denomination is now a localized place name within a townland. It may or may not appear on government maps. However, immigrants to North America used them. They are also found in Irish records.
If you have a Revolutionary War pension file which states someone was born in a place you can’t identify, then you probably are dealing with a sub-denomination. Before your research in Ireland can proceed, you have to locate that sub-denomination is a current townland. Then you can go after the records. The two I start with are:
“Placenames Database of Ireland”: www.logainm.ie
“Index of All Names on the First 6 Inch Survey Maps” on microfilm through the Family History Library: Cork, Clare, Kerry (#2203006); Tipperary, Galway, Leitrim, Mayo (#2203007); Sligo, Antrim, Armagh, Cavan, Donegal, Roscommon, Waterford, Limerick (#2203008); Kildare, Carlow, Down, Dublin, Fermanagh, Kilkenny, Offaly/Kings, Longford, Louth, Meath, Monaghan (#2203009); Laois/Queens, Tyrone, Westmeath, Wexford, Wicklow, Londonderry/Derry.
Be aware, identifying where these sub-denominations are is an essential part of your research. You will need to take the time and thoroughly investigate these. It can literally make the difference between you being stopped in your tracked or proceeding to the next point in your family history.