Thanks for finding the regiment number.
I was able to find his military records as a gunner including discharge in 1919 back to his residence in Liverpool, which may be the same address as the posted letter.
I was also able to confirm his birth date of 17 May 1893 in Durham instead of 1883, as confirmed by his baptism records with parents Michael McGrory and Sarah mcKenna. This then puts Michael A McGrory as aged 23 in the photo, the spitting image of his father too without a doubt. Then Theresa McGrory would have been older still around 28 years old, most likely in this photo as the older of the young ladies.
That then leads to who the others in the photo are. Is the lady to the right of michael then his newly wed wife of Gladys dressed in black surrounded by the mcgrory sisters? Could this photo have been taken around the wedding of 1916 or around the time of 1917 when the letter was posted to liverpool. was this photo taken in liverpool or back in the parents home in armagh.