It's a dangerous business,
walking out one's front door
Allegory: An example of an allegory would be the fight between Smaug and the dwarves. Upon first glance, the fight seems to be about the dwarves desperately fighting to reclaim their kingdom from the evil that is Smaug, however upon closer inspection one can see that the fight symbolizes humanities view of greed. Smaug represents the obvious greed that human society looks down on, the consequences of the fight (the destruction of Lake Town) are a result of a greed that humans try to justify to themselves.
Allusion: Mirkwood (the home of the Wood-elves) was an allusion to Icelandic mythology, as was many of the names of the dwarves.
Dramatic Irony: Thorin and his company had seemingly achieved their goal by driving Smaug out of the mountain. However, that ended up driving Smaug to destroy Lake Town and turning the human village against the dwarves. In the end the dwarves' victory over Smaug was short-lived, as Thorin, Fili, and Kili all ended up dying in the Battle of Five Armies, effectively ending the royal dwarvish bloodline, days after they took back the kingdom.
Figure of Speech: Tolkien uses figurative language throughout the course of The Hobbit. One of the best example would be his use of metaphors and similes in the very beginning of the story to describe Bilbo's home, as well as what exactly a Hobbit is. "There is little or no magic about them, except the ordinary everyday sort which helps them to disappear when large stupid folk like you and me come blundering along, making a noise like elephants which they can hear a mile off.
Mood: The Hobbit starts off with a very lighthearted mood but the atmosphere rapidly becomes darker as they leave the Shire and start their journey to the Lonely Mountain. The characters encounter increasingly dangerous villains as they journey farther and farther away from the Shire, culminating with the Battle of Five Armies.
Reversal: An example of a reversal in The Hobbit would be when Bilbo is separated from the rest of the company in the goblin caves. Because he was separated from everyone else, he ended up finding the One Ring and meeting Gollum. Without the ring the company would probably have never even made it to the Lonely Mountain.