This man spent his military service as JOHN CLARKE and so is "mis-filed" in the WW1 Service Records (WO 363):

Peter MAHONEY, joined the Royal Artillery (in London, as CLARKE) on 25 October 1884, declaring age 24y 10m, born Camberwell. (So born c1860?) Next of Kin: brother JAMES (London). Regt No: 44942.
Not the best behaved of soldiers, he served his 12 years & transferred to the Reserve in 1896, stating an address in SHEFFIELD.

In January 1900 he died (cancer of the larynx) at ROTHERHITHE - as MAHONEY. The death is in freebmd - Q1 1900, age 47y. (So born c1852?)

With so little to work on, he is probably one of those whose descendants will never find him!

Anc****y, CLA, pages 148207 to 148255