I think the joke is that 'ironically' sounds somewhat like 'ionically'.  'Ironically' is used to denote a paradoxical, unexpected, or coincidental situation. An ion is an atom or molecule with a net electric charge due to the loss or gain of one or more electrons. Having a charge means something would be magnetized toward something else with an opposite charge.
' ' It ' s my personal magnetism , ' ' said Tom ironically .