You're absolutely right, Colin.
"Lough Erne" and, appropriately, BCDR No. 30, would be operationally suitable, but would eat more than a smaller loco, and would thus be uneconomic on all but the St Pats & Santa trains. A Jinty or J26 would be better, if available.
There is one very severe gradient in the cutting just south of Downpatrick - in fact, short as it is, one of Ireland's most severe, as the line south of DP was very cheaply built. However, given a decent run, its shortness is such that even a Sugar loco can get three bogies over it.