Quote from Wikipedia: "On the morning of Monday 24 April, about 1,200 members of the Irish Volunteers and Irish Citizen Army mustered at several locations in central Dublin. Among them were members of the all-female Cumann na mBan. Some wore Irish Volunteer and Citizen Army uniforms, while others wore civilian clothes with a yellow Irish Volunteer armband, military hats, and bandoliers. They were armed mostly with rifles (especially 1871 Mausers), but also with shotguns, revolvers, a few Mauser C96 semi-automatic pistols, and grenades."
So as you can see (from this source at least) it was a very varied crowd! There are also some photos online from parades etc. where this lack of a uniform military uniform in the Irish Volunteers is shown.
From what I have read only the Irish Citizen Army wore the slouch hat while the Irish Volunteers had the traditional military peaked cap.