The 14-3 win for the Dragons ensures they closed the gap on Treviso and Connacht - currently tenth and ninth respectively - and McFarland was left to rue a missed opportunity.
"We're extremely disappointed. We were clearly on top of the game for the first 20 minutes and, for one reason or another, the game swung," said McFarland.
"We made a couple of mistakes, and then it was two yellow cards and they scored - it wasn't possible to get back into the game after that.
"They scored that crucial try when we were down to 13 men. That was the turning point and effectively that cost us in the end.
"Building up to next week and our next match [against Edinburgh], we have to pull ourselves back up. That was a huge game for us and the lads are incredibly disappointed.
"Even with the swing, we should have had the composure to win the game. It's going to cost us In the long run. It makes next week's game even more important now."