I understand that this is meant to make Saoirse easier to spell. But in my opinion, Saoirse just looks better than Seersha. Yes, Saoirse can't easily be sounded out to an English speaker, and it doesn't look like any sort of English word, but that is because it is Irish. Not every name will seem English because not every name is English.
I get wanting to make the name easier for people who don't understand it to read, but personally it feels a little inauthentic. I encountered this name once written down, and I thought her parents could have really gone all the way with this one, but they chose to go with the safer option, which is kind of underwhelming. The name is still pretty though, and I don't hate it.