There were 8 "Thomas Murphy" in the RMF
Of the 8 Medal Card Index (and it is to all intents and purposes a complete record) men with this name on the MIC , only one is listed as a sergeant (there is a corporal as well, and possibly him as many men had temp or acting ranks in WW1)
Of the 8 Thomas Murphy in the RMF, 4 are listed as killed in CMGC (they are all listed as Privates)
The sergeant on the MIC is 3974 Sgt Thomas Murphy. He went to France in Dec 1915 (from memory that would have to be with 2nd battalion, as 1st was in Gallipoli, and I do not think any other RMF were in action at that time in France), but it is entirely consistent that a man might return to Dublin, perhaps wounded, and spend a while training new men there, before being sent back to an active role.
That man did not get the SWB, so the implication is that he survived the war without being invalided out.
If he was sent abroad in 1916 it would have been either to the Middle East with the New Army Munster battalions, or to France with the 1st or 2nd battalion.
The service record for the above Thomas does not survive (you can check Ancestry records)
My feeling is that you need more family info to get any further. Basically each battalion acted as its own little regiment, so to know the battalion is essential in order to know where he served.
Does nobody in the family possess for example the actual medals. Did he get any gallantry awards - in which case the citation would/could tell you where he was at that time.
One needs a fair amount of persistence to track these things down!