Watch the video footage.
Macarthy looked to see Rooney coming and changed his direction to block him, he also then at the last minute put his shoulder in. If you watch Rooney he tried to take some evasive action, a sort of side step and swerve move, and yes the elbow came up, but it wasn't a swinging elbow and half caused perhaps by him running at full tilt on a line that was open to him and then having to imbalance himself.
You may think a red, I would suggest a yellow at worst but I would also say that the cause was an illegal block by Macarthy - but of course some people are always ready to get stuck in to him.