While most silversmiths made a bit of everything, James Kennedy is said to have been a box specialist but on those he used I.K (I have a couple of his snuff boxes) so while Bennett lists him under IK as well, I'm not sure of the reason for this. Auctioneers often incorrectly attribute snuff boxes with I.K to James Keating but he appears to have been more involved in making tableware.
In Jackson's Silver & Gold Marks, John Kelly's IK mark is noted against table spoons while John Kavanagh is not mentioned at all (far as I can find) but that doesn't mean this IK mark on the fork is John Kelly's and not John Kavanagh's.
Intriguingly, Jackson mentions another maker who also used IK and that was John Kenzie. His name and mark appear on a plate of pewter preserved at the Dublin Assay Office, displaying marks used from about 1765 to 1812, but I know nothing else of him or what he made.