A nice write up of the involvement of William/Liam O'Brien and his comrades in Cork during the Easter Rising appears on this post by Ruari Lynch
http://diarmuidlynch.weebly.com/irish-volunteers-officer-training-january-1916.html#
William/Liam was more active in 1916 than I had originally understood.
He died not too long after his release from Frongoch, having previously been a healthy and sporting individual