Speaking the last line: definitely an irish thing, as Jim C has said.

Cupping your ear means you may just hear your own voice slightly better by AIR conduction.
Putting your finger in your ear means you hear your own voice by BONE conduction (as well as obliterating extraneous noise).
Quite hard to do either if you are playing an instrument and singing at the same time, so how DO we manage, I ask myself?? (and therefore, why do non-instrumentalists have to do it at all!!??)