It was definitely against Farrell. Impossible to tell from the pictures as there's about 3 seconds between Paddy Barrett knocking it forward and Farrell appearing in the picture. You can certainly see he's come from closer to the goal to win the header. Andy Boyle seemed fairly convinced he was offside and is appealing as he's going for the header and then gives the assistant a clap for spotting it. Boyle, as a defender, would do that but if there was no chance of him having been offside he wouldn't have been appealing at all.