Part of what makes Dogberry such a humorous character is the irony Shakespeare employs in Dogberry's characterization. Dogberry is a poorly educated individual, as seen in his poor use of language, but he is also a very prideful character. In fact, Dogberry is a bit of an idiot and does his job as Constable very poorly, but also feels an ironic sense of pride in what he does, unable to perceive that he is doing his job very badly.
As part of his poor use of language, we see Dogberry mixing up words all throughout the play and his partner Verges follows suit. A few examples can be seen when we first meet both characters. Dogberry is asking the men he has selected to guard the prince if they will be honest and loyal servants of the law and Verges attempts to threaten them if are not honest and loyal by reminding them that if they are not, they might "suffer salvation" (III.iii.2). Of course, Verges actually means "damnation," or eternal judgement of the soul, should they prove to be dishonest. Dogberry continues this mix-up of words by saying that the punishment of "salvation" would be "too good for them, if they have any allegiance in them" (4-5). Here, Dogberry not only mistakes the word "salvation" for "damnation," he also mistakes the word "allegiance" for treachory, or "disloyalty". All of these word mix-ups show us that Dogberry considers himself to be far more educated and intelligent than he actually is. This use of irony helps to relay Dogberry as a very humorous and entertaining character.
This pep talk reminding his watch to be faithful and honorable men also portrays Dogberry's pride in his job as Constable. It is quite clear that Dogberry sees himself as an honorable man, otherwise he would not be reminding his men to follow suit. However, the irony in this scene, as well as in Dogberry's character, is that Dogberry actually performs his job as Constable quite poorly. We especially see one instance of this when Dogberry advises his men not to "meddle" with any thieves they spot for meddling with thieves may compromise their own honor. Dogberry is confusing the act of meddling, or interfering with thieves, with the act of working along side of thieves, as we see in his line, "[T]he less you meddle or make with them, why, the more is for your honesty" (48-49). When Dogberry is asked by his men if they should arrest thieves, Dogberry further portrays his poor qualities as Constable by advising them to prove that the thieves are thieves by allowing them to "steal out of [their] company" (55). Later, when Borachio and Conrade have been caught and Dogberry is attempting to interrogate them, the sexton even points out how bad Dogberry is at his job by pointing out that Dogberry does not understand how to carry out an interrogation, as we see in his line, "Master Constable, you go not the way to examine. / You must call forth the watch that are their accusers" (29-30). All of these examples of Dogberry's poor performance in his job as Constable helps to point out that, while Dogberry is very proud of his post, he ironically performs it very badly.
Hence, we see that it is the irony behind Dogberry's characterization concerning his poor education and the poor performance of his job that makes his character such a humorous and entertaining character.