I assume you already have the two page record of his discharge which is on FindMyPast. Just a couple of corrections to what you have said so far. He wasn't discharged
from the Kilmainham Hospital but
to it. Also the mention to his trade as a blacksmith refers to his civilian occupation. While he may have put his smithing skills to use in the Army he wouldn't actually have been employed in his battalion exclusively as a blacksmith.
I doubt if there are any parts of his service record which have survived.
My guess is that he joined the second battalion of the 58th. The 1st Battalion was in Gibraltar, followed by Malta, then Sicily and Italy during the period 1805 - 1808, and then took part in the early stages of the Peninsula War before being sent to Canada to fight in the American War of 1812. The 1st Battalion then returned to Britain, via the Netherlands in 1815. It's not impossible he served with the 1st Battalion, but as the 2nd Battalion was raised in Ireland from 1804, and then deployed to Portugal in 1809 to take part in the Peninsula War, I think this fits better with his dates. After the defeat of Napoleon at Waterloo in 1815, the 2nd Battalion returned to Ireland and was disbanded there in December. While some of the men of the second Battalion were sent to join the 1st Battalion at this stage, this would have been an opportunity to discharge any soldiers who were less than fully fit. Plus George was aged nearly 29 years which was relatively old for a soldier. I wouldn't be surprised to find that his epilepsy was a convenient excuse to get him a pension, which is why he came under the care of the Kilmainham. This was the Irish equivalent of the Chelsea Hospital - not so much a hospital, more the administration of pensions.
https://en.wikipedia.org/wiki/Royal_Hospital_KilmainhamYou can find out for sure which battalion he was in by searching the muster rolls at TNA. Unfortunately these haven't been digitized so you would need to visit in person.
The 1st Battalion muster rolls for 1808 - 1810 are in
WO 12/6721 and 2nd Battalion rolls for the same period are in
WO 12/6783