Retailers mark of Keene on a George Nagle teaspoon hallmarked at Dublin in 1815.
This is likely to be John Keene II, the son of John Keene, the former Master of the Dublin Goldsmiths Company in 1801.
John Keene II was apprenticed to his father in 1802, and it would appear that he took control of the firm following the death of his father in 1807. It is somewhat doubtful if he completed his apprenticeship, he could not have commenced the seven years of learning the trade until the later part of 1802 as his father, as Master, was not allowed to practice his craft whilst serving in the position of Master of the Company, and it would be an unlikely scenario that he would have been bound to another master to complete his apprenticeship and run his own company at the same time.
This is only the second example of Keene's mark that I have noted, both have been on George Nagle spoons, the previous one hallmarked in 1813.