Often times, people make the mistake of considering sunk costs, or costs that have already been paid, when making decisions. But sunk costs are irrelevant because there is no way that the cost can be recovered.
My economics teacher says that it is easier to think of irrelevant costs as unavoidable costs. Any cost that is unavoidable is irrelevant to whatever decision that is being taken. This is how I remember the difference between irrelevant and relevant costs.
We have to concentrate on what's relevant or avoidable. If there is a cost that has already been paid out or if there is going to be a cost in the future that cannot be avoided for whatever reason, it is irrelevant.