Captain George Norman Roberts, Royal Cork City Artillery, appointed as a Lieutenant in the 2nd Battalion [sic. Regiment], Natal Native Contingent vide District Order No. 9, dated 26th April 1879.
The 2nd Regiment N.N.C. fought at the Battle of Ulundi, under the command of H. M. Bengough, 77th (East Middlesex) Regiment.
No trace of any wound sustained by Roberts in action, two officers of 2nd Regiment, N.N.C. were slightly wounded. One of those being Henry Timson Lukin, a very well-known name in South African military history.
His brother was Dr. Frederick Joshua Roberts, who as you say had the medal with no clasp, which rather precludes him from being present at Ulundi.