Translation according to P. W. Joyce:
Coolagh, the name of more than a dozen places,looks simple, but there is often much doubt as to its exact signification. The weight of the best local Irish speakers favours Culach (cul joined with the termination ach), meaning a back place, such as land at the back of a hill. Coolaghy in Donegal, Tyrone,and Queen's Co.; cul-achaidh, back field : see Agha [reproduced below].
Agh and Agha, whether in the beginning of names or elsewhere, generally represent the Irish achadh [agha], a field, of which it also conveys the sound: the gh has a guttural sound. But Agh often stands for ath [ah], a ford, of which however it is not a good anglicised form. See Aghaclay below. It will be observed that Agh and Agha, meaning a field, are far more common in the north than in the south and middle of Ireland. Aghaclay (better Aghnaclay), near Clones, co. Monaghan, so called from the old lord made of hurdles (ath, ford; cliath [clee] hurdle), like the original hurdle ford that gave the same name - Ath-Cliath, hurdle ford - to Dublin. Aghnaglea, in Tyrone, is the same name, with the c eclipsed by g in gen. plural (p. 3): the ford of the hurdles: all illustrating how usual was this mode of bridging over dangerous river fords.