Many Irish Volunteers are seen wearing Irish Free State floppy caps which weren't worn until 1922 by the Free State / National Army.
Some Irish Volunteers are seen wearing black RIC bandoliers instead of the brown ones actually worn by the Irish Volunteers.
At 39:11, when Arthur Mahon aims his rifle at his brother, coming down the stairs with Elizabeth, his bayonet is clearly not affixed to the rifle muzzle properly, and drooping down.