Hi Bill
Thanks for that - from your info, and what ShaunJ said, it seems he wasn't with the regiment in Afghanistan, but with the Depot Brigade in Clonmel - perhaps due to his age and/or physical condition. If his family had been back in Ireland, perhaps this is another reason he returned home.
If he served in the Afghan War he would have been in receipt of a medal. There were some regiments that garrisoned Kandahar after the end of the war and didn't get the medal, but this was end of 1880 into 1881.
The 18th arrived in India in 1874, and moved into Afghanistan in January 1880 - some men may have been sent back to the Depot HQ at this point. They returned to India in March 1881, and to England in 1885.
All best - Garen