From the first marriage of her son Thomas in NY, his parents were Thomas and Ellen Schrows (which I knew spelling of Ellen’s last name was incorrect). Since there are no Shruhan on Griffiths Valuation, is the name last name Shruffin instead.
There was no right or wrong way to spell a surname. If someone couldn't write or spell s/he spoke the name to the person who was doing the writing. The scribe then wrote what he or she heard and if it was a name they weren't familiar with they spelled it however they decided it should be spelled. A priest might spell a baby's surname one way, a registrar might spell the same baby's name a different way. If a new priest arrived in a parish and baptised that baby's siblings, he might spell the surname a different way, especially if he wasn't local. Ditto mother's maiden surname. If the family moved to a different parish or a different district, priests and registrars there might have their own versions of how it should be spelled. e.g. Doherty, Dougherty, Docherty and Dockerty are all the same name. My Roscommon aunt's family were Mulvee/Mulvie/Mulvey &c.
Even when people could write, there were instances of surname variants within a family:
A man went to register his brother's death. He spelled his brother's surname for the registrar. When information had been written down the man signed, spelling his surname differently to the way his brother's was spelled. When the registrar queried it the man said that his brother consistently spelled his name one way while he preferred a different spelling.
A household census return has a family surname spelled 2 ways. 1 person filled in some entries and another person then completed the form.
Another thing to keep in mind is that many surnames were originally Irish. The Irish alphabet has fewer than 26 letters. There are sounds that an English-speaking person may have had difficulty interpreting into writing.
Some people altered their surnames to make them seem more/less Irish, depending on their situation or personal preference. For some it may have been to make them easier to spell.
