A vagina that is suitably irrelevant and therefore refered as such. The word irellavagina can be used to replace other words in a scentece when one can't think of the right word, or is just too lazy. For example:
"I just went and picked up some irellavagina. We were all out."
"I don't know what do to! Maybe I'll just irellavagina.
"God, Katie, could you be more of an irellavagina?"