It would not have. Shiel had the ball. When you have the ball, it is not a bump. The onus is on the tackler to not knock themselves out in the tackle.You are 100% correct in that there are occasions when a head collision is purely incidental and the view as espoused by some that players should not elect to bump or spoil is crap. Last Saturday when Connor Rozee was flattened Dermot Brereton was full of, 'how great the body contact was', yet if Rozee had hit his head on the ground when he fell and had a HIA the incident would have been looked at by the tribunal.
Otherwise Barrass would have been cited for causing Marshall's concussion when Marshall knocked himself out trying to tackle him.