I watched the game live on Jagzone and was as disappointed as everyone else that Doolan was flagged for offside, but unless you were in the same position as the A/R and watching the game the WAY the A/R was watching it, then it's difficult to comment.
Firstly, the camera angle made it impossible to say if he was offside or not.
Secondly, the majority of spectators and those on the bench would have been watching the player in possession of the ball when it was played forward to Doolan, and not watching Doolan. The A/R would have been watching the second last defender, and from his position it would be relatively easy to assess Doolan's position. He would also have been using his peripheral vision to watch the attacking player in possession of the ball and know when it was played. If your focus is on the attacking player only, then in the split second it takes to change your view, the offside player can appear to be in an onside position.
I make these comments based on my own experience as an A/R at a fairly senior level for over 15 years.