Ok well I think that's then a case of differing opinions. The question is why he put the arms up? Looks clear to me it was because O'Sullivan was about to run straight into him at full pelt. What would you do if you were in that situation? I'd put my arms up to protect myself, shield, deflect, absorb the impact. Probably close my eyes and wince too, even though I'm not nearly as tough as GB.
Dec OS is probably about the same height as Brennan. If both are standing to their own height, which one player running and another standing/getting ready to block or tackle usually would be, then Ger's raised arms wouldn't collide with Declan's head, rather his chest and/or his arms.
So why did it pan out the way it did? Because at the last fraction of a second (look in real-time) DOS goes downwards, with absolutely no chance of adjustment by Brennan. He goes downwards because he tries to stop himself colliding at full pelt, by digging his boot(s) in, leaning back a bit, and probably slipping.