How is the following quote ironic, and what theme does it show? "We've got to have rules and obey them. After all, we're not savages."

In chapter 2, the boys fail to make a sustainable signal fire, and Ralph mentions that they need to put people in charge of maintaining the fire at all times. Ralph also says that they should have more rules and Jack agrees by saying,

"We've got to have rules and obey them. After all, we're not savages. We're English, and the English are best at everything." (Golding, 33)

Jack's comment is ironic because as the novel progresses, Jack and his band of hunters reveal their savage, primitive nature. Jack and his hunters paint their faces and become bloodthirsty barbarians as they hunt pigs, murder Simon and Piggy, and even attempt to kill Ralph.
After landing on the island, they rapidly descend into savagery and live like uncivilized beings. Jack's comment also emphasizes the theme of civility vs. savagery. As English schoolboys, Jack and the others have been taught that they are inherently civil and superior to other races, ethnicities, and cultures. However, Golding portrays all humans as inherently evil, wicked individuals without the restraints and influence of society to prevent them from acting like savages and indulging in their true desires.