The Linnet's comments about the Miller are marked by verbal irony throughout the story. He repeatedly refers to the character as a good and devoted friend, yet he only ever gives examples of the Miller behaving in a selfish, unkind manner. The Linnet's verbal irony ensures that he is able to hint at his thoughts on the events of the story without expressing them explicitly. Although the verbal irony gives an idea of where the Linnet's sympathies lie, he never inserts his personal commentary into the story he is telling.
For example, when the Linnet first introduces the Miller into the story, he claims that he is Hans's most devoted friend. However, the next piece of information he gives—seemingly to back up this claim—would only attest to the opposite: that the Miller does not treat Hans well at all.
Little Hans had a great many friends, but the most devoted friend of all was big Hugh the Miller. Indeed, so devoted was the Miller to little Hans, that he would never go by his garden without leaning over the wall and plucking a large nosegay, or a handful of sweet herbs, or filling his pockets with plums and cherries if it was the fruit season.
The first sentence of this passage sets the reader up to expect that the Linnet will describe the various ways in which the Miller acts as a devoted friend. When the Linnet immediately disproves his earlier claim in the following sentence, the reader understands that the bird is most likely narrating with a subtle kind of irony. He calls the Miller Hans's most devoted friend as a way of mimicking the Miller's own words about himself, all while revealing the ways in which he is everything but. After the reader has been given a stirring description of Hans's beautiful, abundant garden, the image of the Miller leaning over the wall and stealing flowers or filling his pockets with fruit feels like a particular encroachment on the honest little Hans.
Throughout the fable, the Linnet never directly articulates his own personal thoughts or value judgments on whether the Miller is behaving justly towards Hans. This lack of explicit input in the story highlights the Miller's insolence in a unique way, as the indirectness of the Linnet's verbal irony forces the reader to reach their own conclusions without the influence of already-elaborated value judgments. Verbal irony allows the Linnet to express his opinions while also ensuring that the narrative remains free from the storyteller's straightforward opinions.
Depending on how one approaches the story's title, it's possible to read it as containing a layer of verbal irony as well. It's unclear whether the "devoted friend" in question is Hans or the Miller. If the title is pointing to the Miller, it would be marked by irony, as the story only reveals him to be a selfish, evil-spirited person in his relationship with Hans. If the title is pointing to Hans, the title wouldn't be ironic, because Hans really is a devoted friend—so much so that his devotion has fatal consequences. In either case, the word "devoted" in the title contains connotations that are hidden from the reader at first glance. A word that tends to contain positive connotation, it comes to be stripped of this by the end of the Linnet's story. One of the characters is too devoted, the other character is not devoted enough.