He would be Ethel yes. At Waterloo, there was the British Army, Hanoverian and German contingents, Dutch and Belgian contingents and the Prussian Army (and the French of course). I can't conceive of him being in any other army than the British.
The British army at the time was full of Irish and Scots soldiers (and not only in the specifically Irish and Scottish regiments). Every regiment in the army was a melting pot.
I'll see what I can find. Just one more question, was he born in Ulster or Scotland? Do you know exactly where?