The feud between the Grangerfords and the Shepardsons is a throw back to the absurdities of Romantic literature and the fashions of the time period. Twain pokes fun at elaborate Victorian decoration of the household, the overzealous melancholy of Victorian literature as portrayed in Emmeline's poetry, and the ideals of "honor" respected during the time but foolishly clung to by the two families as a reason to continue their vicious fighting.
The simplest irony is this: the two families don't know why they are fighting. They've forgotten. Second, the two families arrogantly assert their superiority, but neither knows of the secret relationship between Sophia and Harney. Finally, there is irony in Huck's admiration of the home. He is in awe. It is the first real family home he has ever seen. It is, however, a dysfunctional home, as Huck will find out upon Buck's death. It is another disillusionment of Huck, another stab at society.