I am not so sure about "the never mention hobbies that are completely irrelevant".
I would say that if you are researching your family tree then put geneolgy down as a hobby no matter how irrelevant it is to the job. It's a popular and inoffensive hobby and there is a very good chance that one of the interviewers is also interested in geneology.
Just make sure that the geneology part is kept short and put after any hobbies that are relevant to the job.
It's stuff like stamp collecting that is taking the risks and may make you look like a tosser.