John Duffy

On 16th July 1858, age 17, John Duffy enrolled as a volunteer for the Militia. He was a labourer born in Kilbride, Tullamore, in the county of Kings (Offaly). Then two years later he transfers to the 21st Foot Regiment.
On 23rd October 1860, in Tullamore John Duffy records he is 19 years, a labourer, from Kilbride, Tullamore, County Kings. At the question of have you served in the Army, Marines etc. it states
Yes in The Kings County Rifles for 2 years 3 months. John enlisted for ten years. It looks like he served over 20 years, at least until 3rd November 1881.
John served in the East Indies and the West Indies. He was at home between 13th November 1870 and 3rd November 1881. His cause of discharge was ‘In consequence of having claimed it on termination of his second period of limited engagement’.

I got this information from the military records on FMP.
What I want to know is can I get any further information? I would be particularly interested in whether The 21st Foot Regiment was in Penally, Pembrokeshire in the early 1870's.
Can anyone suggest anything to help my quest?
With many thanks
ELMA