The Royal Artillery unit he joined was probably the 56th (King's Own) Anti-Tank Regiment, Royal Artillery, which if he stayed with them was sent to India in 1942 to fight in the Burma campaign. If he fought in Burma he would have received the Burma Star and so you could also contact the
Burma Star Association to see if they have any information about Joseph P Devlin. He doesn't seem to have been a member of the association but that doesn't mean that they won't hold any information about him, if some of his mates joined and he featured in any of their accounts.
However after it was formed the 56th AntiTank Regiment split off some of its men to create the 66th Anti-Tank Regiment which remained in the UK and acted as a reserve pool of trained manpower to reinforce other units as required. The only way you are likely to get the full story of what Joseph did during the war is through his service record.