The Monmouth-class ships' main armament consisted of fourteen breech-loading (BL) Mk VII guns.Friedman 2011, p. 81 Four of these guns were mounted in two twin-gun turrets, one each fore and aft of the superstructure, and the others were positioned in casemates amidships. Six of these were mounted on the main deck and were only usable in calm weather.Friedman 2012, pp. 251–252, 260–261 Ten quick-firing (QF) 12-pounder () 12-cwt guns"Cwt" is the abbreviation for hundredweight, 12 cwt referring to the weight of the gun. were fitted for defence against torpedo boats. Donegal also carried three 3-pounder Hotchkiss guns and two submerged 18-inch (450 mm) torpedo tubes.
