Your best bet might be head swaps between Brit bodies and German heads from any Great War range. Ok the collars aren't right and IIRC the Irish tended to wear leather leggings rather than puttees but if you aren't too obsessive..... The German style Vickers helmet, '08 webbing and SMLE are probably the most defining aspects anyway.
Of course your other option would be to do an Eoin O'Duffy style fascist Irish army. Famously or rather infamously, his Blueshirts participated in the SCW on the side of Franco and the nationalists, although without any particular distinction.
For Option 1, consider Gripping Beast/The Woodbine Design Company's "Great war in the East" range - British in serge dress, use say the Ottoman stahlhelm instead of British heads, bit of greenstuff over the puttees to turn them into leather leggings, and fiddle with the collars if you want to be really obsessive. However, as this is a "fantasy" Irish army from the 1930s, it hardly matters what the collars looked like.......

As for Duffy's Incompetents, Empress Miniatures' Spanish nationalist regulars as a sort of "Irish foreign legion" would be a good fit. Given the berets worn by the Blueshirts, I think the Carlist berets might also be a nice touch.