An irrelevant cost is a cost that will not change as the result of a management decision. However, the same cost may be relevant to a different management decision. Consequently, it is important to define those costs that should be excluded from consideration when reaching a decision.
For example, the salary of an investor relations officer may be an irrelevant cost if a management decision relates to issuing a new product, since dealing with investors has nothing to do with that particular decision. However, if the board of directors is considering taking the company private, then it may no longer need an investor relations officer; in the latter case, the salary of the investor relations officer is highly relevant to the decision.
Sunk costs, such as the purchased cost of a fixed asset that was incurred in a prior period, are also usually considered irrelevant when making decisions on a go-forward basis.