At the break out of WW1 the thoughts of republicans were confused by the thoughts of achieving Home Rule which was almost finalised. Many men, especially in Dublin following the lock-out, saw the army as a wage and means to achieving Home Rule; it was definitely not seen as an event going to take four years. They were fighting for the "little countries" of Belgium, etc. including ourselves.
When the war dragged on and Home Rule was not forth coming, 1916 and more radical republicanism began to appear. It was seen that the war was being used as an excuse for not granting Home Rule at the time.
My OH's grandfather was a staunch republican with a scantily documented history in War of Independence and Civil War; but we have his pension application and some other documents about him.
However, he served with the British Army throughout WW1 as he joined because of what was called "Redmond's Call", he joined in late 1914, at the very beginning of the war, believing that it "would be over by Christmas" as many others did. He travelled with the army through Europe and the Middle East for the full duration, returning to Dublin in late 1918.
Only his medal card survives in the British records as many records were destroyed.
I would not rule out that you Frank was not in the army for a period