Hi Gerry,
To answer your question, your Grandfather is wearing the uniform of a member of the Ulster Special Constabulary, by the date he may well be in the 'A' category who were predominately ex-servicemen and recruited for their skills as soldiers, but only with exemplary service records! They would have served on a full time basis in support of the RIC/RUC. The 'A' class of USC constables were disbanded in 1924/25, but a lot of these men were absorbed into the RUC.
His record will be in the PRONI archive, but you may find it's only two pages in length.
With your permission can I use this photograph for a website I'm putting together for the USC association?