To those of you who see this, I need your assistance. Included in the effects of Laurence Cahill was a Royal Irish Constabulary sword. How did this sword come to be owned by the Irish born Lt of the 69th New York Volunteers. Did he travel back to Ireland after the war?

This RIC sword and scabbard were the property of American Civil War officer Laurence Cahill who fought with the 69th New York of the Irish Brigade. Cahill served with the 69th NYSM and later with the 69th NY Infantry. He suffered a compound fracture of the tibia by a musket ball wound and was captured at Malvern Hill on 1 July 1862. Cahill was later exchanged and discharged because of his wound. He then served in the 2nd Battalion of the Veteran Reserve Corps, and rose to the rank of Brevet Lt. Colonel before being discharged July 20, 1866.He was still alive in New Jersey as he swore an oath on a document regarding his pension on March 22, 1871. Also included in the collection from his family is a letter from a library in Jersey City New Jersey to Cahill's granddaughter regarding the loan of the sword, cane and general effects of Laurence Cahill for a display in 1918.

I have been unable to find any correlation between Cahill and The Royal Irish Constabulary. Is it true that the the term "Royal" was added in 1867.The ricasso of the sword seems to be marked in 3 lines. The first line is SEXTON. The second line appears to be DAWSON ST. The last line in not discernible.  The etching of the blade of the sword is marked Royal Irish Constabulary. Does anyone know when these types of swords were issued and to whom? 

Might anyone have a link to which you could refer me to research this more in depth?