When is a Supreme Court decision irrelevant?

A Supreme Court decision is only irrelevant in two ways. The first is "de facto". This is when a decision is obviated by the fact that it no longer has any practical meaning. For instance, let's suggest that the Supreme Court once ruled on the use of debt sold by the government from 1800 to 1810. Now in 2008 that debt has either no value or no longer exists, so the law pertaining to that simply does not matter. De facto is latin for "in fact" or "in practice".

The more common and controversial way is when the Court overrules a previous decision. This happened with Plessy v Ferguson and then Brown v Board of Education. In Brown v Board, the Court decided that its earlier decision of Plessy was a mistake so it was struck down and thus became irrelevant.