I did a quick search for a Philip Gray's medal card on the National Archives database.
http://www.nationalarchives.gov.ukOnly 11 names came up. Only 1 of those in an Irish regiment.
Description Medal card of Gray, Philip
Corps Regiment No Rank
Royal Irish Rifles 10848 Private
Royal Irish Fusiliers 51865 Private
Date 1914-1920
Catalogue reference WO 372/8
If this is him, you now have his regiment and number. Invaluable for tracing his records. As he survived the war there is also a better chance of his records being available at Kew.
You'll need to take a look at the other Philip Grays listed. I may have the wrong one here.
Just go to the National Archives page.
Click on Military History/British Army and find the links through to the War medals Index cards. you can view the card for £3.50 and that holds clues to where he served. (It'll need deciphering !)
I'm sure there will be others here who can be of more help.
best wishes
Lizzey