WW1 Volunteer Training Corps. Think of the Home Guard but raised locally rather than by the War Dept. They wore their own uniforms in 1915-16 before adopting standard khaki when they became Volunteer Battalions (not be to confused with pre 1908 VBs) of the local Infantry regiments. Your man is wearing a VTC style jacket (they had several styles) and cap badge .
VTC cap badge designs are not recorded that well and there are lots of unknowns. There are some excellent albums on the Forum which may help.
Despite a few issues in Ireland with arming the locals there was an Irish Association of VTCs. See this article from 1916:
http://archive.spectator.co.uk/artic...ing-corps-to-t