Can anyone explain the spelling behind this? It almost follows Irish spelling rules (and this name is very nearly Sinéad) but it was my understanding that Welsh does not have the same broad/slender split that Irish does. Is S followed by I also pronounced [sh] in Welsh, or is this name unusual in that regard?
The Welsh 'si' is just like the pronunciation of a few words of the Irish language. Some words that start with an 's' can make a 'shi' sound; and will be followed with an 'a', an 'e', or an 'i' and maybe even a 'u' (and possibly a 'y') might manipulate the pronunciation of the Celtic linguistic word.