Irrelevant reasoning fallacies are those that take place when an arguer introduces a premise that, when you think about it, is utterly beside the point.

This may be the most common type of fallacy, because often the arguer is convinced it's relevant, too, and both sides accept it as a good point. The problem is that it can lead to false conclusions.

The most common type is probably attacks on the arguer. If a person advises a certain drug for a condition, we can argue that they're on the take from the pharmaceutical company. This allows us to avoid having to decide if it's the right medication based on understanding pharmacology, which most of us don't have.

Another example of this is when a prosecutor is told he can't introduce the accused's previous convictions as evidence that he might have done the crime in question. Really, the question is did he do it this time, and the only result would be to prejudice the jury. So, it's not allowed as evidence, as frustrating as it may be.

One way to identify this class of argument is to consider whether the premise is true, and then consider whether its truth affects the outcome of the argument. For example:

P1. The capital of British Columbia is Victoria.
Conclusion: pi is close to 22/7.

It's true that Vancouver is the capital, but so what?