I declare an interest: both my grandmother and my aunt were called Irene, with the final syllable pronounced, but throughout my grandmother's life she found herself being called the short version. Born and raised in Devon where no-one had a problem with the three-syllable version, she spent much of her life subsequently in south London where everyone assumed it was "Ireen"...
I grew up with a woman named Irene (een) down the road and Irene (eenee) Handl on the telly. So I always knew both existed. Took it for granted. One of those things that make our language such a rich tapestry blah blah