what is ironic about they found it comforting to be reminded that,after all,they were truly their own masters and that the work they did was for their own benefit?

The first two lines of chapter nine are:

"Boxer's split hoof was a long time in healing. They had started the

rebuilding of the windmill the day after the victory celebrations were

ended."

I'm not sure these lines relate directly to your quote. In general they were not their own masters. One human dictator had merely been replaced with a worse animal (Napoleon) dictator.