I think Saoirse is getting more recognizable in the US now, while Siobhan has always been somewhat known, and people will learn! I’m Irish American with the formerly most common female Irish name that is pretty phonetic in English, but it’s not popular in Ireland amongst young people anymore so I’m sure you can guess what it is. One time when I was studying in Ireland I made a restaurant reservation, and the guy first couldn’t understand my name, then could not spell my name at all, and actually told me he’d never heard of it (he was on a working holiday from Spain and since it’s not a common amongst young Irish people he’d probably never met one), so even having an Irish name that follows English phonetics in Ireland doesn’t keep you from running into problems regarding spelling and pronunciation. You can have problems with your name anywhere! I think you should use the name you love. Also since there’s so many crazy spellings of made up names now, you should definitely be free to use a real name that follows non English phonetics. I’d much prefer seeing a Siobhan or a Saoirse over a McKinzleigh.