A series of cores from Carboniferous rocks in Co. Donegal, Ireland, revealed the presence of limestones, dolomites and dedolomites beneath the Cliff Dam. The conversion of limestone to dolomite probably occurred early in the rock's history, assisted by the presence of evaporite minerals such as gypsum. The conversion of dolomite to dedolomite was probably a later event related to fault-assisted ingress of meteoric waters during uplift of the rock mass. The compressive strength of the limestone is considerably affected by both processes. In this case, the conversion of limestone to dolomite resulted in an approximately 20 0ecrease in compressive strength whilst the limestone to dolomite to dedolomite conversion resulted in a 75 0ecrease. Thin-section study may be necessary in such cases to distinguish dedolomites from their associated country rocks.