Thomas McPhee joined the Royal Munster Regiment on 8 September 1914, so very soon after the start of the war, as part of the Kitchener New Army. He would have volunteered and was not conscripted, so he would have had a choice about the Regiment which he joined. Therefore if this Thomas McPhee is the one who was born in Govan, then I would expect that he may have had a family connection with the Munsters, or perhaps they just happened to be actively recruiting in Glasgow at around that time.
It appears that he joined the 6th Service Battalion and went initially to Gallipoli, arriving there on 7 Aug 1915. But as the Gallipoli campaign wound down, the battalion moved to Palestine where they remained until March 1916 when they were moved to France, and the majority of the men of the 6th Battalion were used to reconstitute the 2nd Battalion. However if the hospital records are correct, Thomas was not one of those who transferred at that time. In early 1918 while still in B Company 6th R Munster Regt, he was twice admitted to 19th General Hospital, firstly suffering from scabies, and then a boil of his right buttock. From hospital he was sent back to the Command Depot and it was probably at this point that he was transferred to the 2nd Battalion R Munster Regt, and was with them during the so-called 100 day offensive which started in September 1918, and lead to him being shot in the hand during an operation to capture the town of Catelet. He was hospitalised on 5 Oct and probably did not return to his unit before the Armistice was declared on 11 November. As you know, he was discharged from the Army on 31 Jan 1919 as being no longer physically fit for war service. In addition to the 1915 Star and the British War and Victory medals, he also received a Silver War Badge No. B145002.