I can not say with any degree of certainty when exactly this button was modified. However, it looks to be quite old and not something that was done recently. The button dates from the World War One era but no later than 1953.
The button itself is a large brass other ranks type to the Irish Guards. The button has the maker’s mark “Wm Dowler & Sons Ltd BIRMm.”
A coppery bronze “IRA” lettering is (braised or soldered)onto the King’s crown with the Maid of Erin harp clearly visible below.
A very unusual piece, the first I’ve ever seen with this type of modification.
Please click on the images for greater detail.