This is a very scarce button and by it’s design quality most certainly is an officer’s button.
With a central shamrock, the circumference of the button is marked SOUTH – IRELAND – YEOMANRY
The reverse of the button is maker marked “JENNENS &Co LONDON”
This is a standard size button for an S.I.Y tunic, it measures 20mms in diameter.
The button is in very good condition.
I have a small and limited number of these buttons in stock.