These marks are to be found stamped on the top of gun barrels of arms registered under the Arms Act of 1843, County letters followed by a numeral or numerals.
The letters were appointed by the Chief Secretary Ireland. (These marks are not to be confused with those found on Irish Police arms, with prefix letters C., R.I.C., and R.P., followed by numerals, as these stand for: Constabulary - Royal Irish Constabulary - Revenue Police).
Ref - Arms Act Ireland, 1843 (6 and 7 Victoriae Cap., 74)
The Journal of The Arms & Armour Society. Vol. VI, No. 10. June, 1970. (Police Marks on Arms of Irish Provenance by Oliver Snoddy)