Okay, so the original Irish spelling of ‘Shannon’… which is it? Are the two 'n’s in the middle or at the end?! Different websites from google seem to use both interchangeably?
E.g. there are both spellings in this one article: Shannon (given name) - Wikiwand
I’d be so grateful if someone could help out!
From what I can find, Sionann was the name of the figure from Irish legend, Sionainn was the name of the river her body was left in - so I’m guessing two Ns at the end?
[name_m]Ah[/name_m], darn. I thought I’d just sorted it out from googling away that most Irish people with the name seemed to have two n’s in the middle 
I wonder if both are acceptable?
We may not even have any more kids and if we did we might not have a girl, but I can’t help planning what with my name addiction! It also seems silly to use the Irish spelling when kiddo would be only like 1/8th or so Irish and not raised in [name_u]Ireland[/name_u], but it would be for middle not first, and I find the spelling so much more aesthetically pleasing somehow?
I think both are probably acceptable and it will be a regional thing maybe? (Kind of like how [name_m]Calum[/name_m] and [name_u]Callum[/name_u] are both acceptable?) Still, that was just what I found from a five minute search, so it won’t be perfect. If you like the two Ns in the middle, i say go for that 
Disclaimer: I am definitely not an expert in Irish names!
Generally, if you have any questions about Irish names, I would refer you to the Irish government’s official website for baby names: Interactive Data Visualisations | CSO Ireland
I’ve never encountered anyone named Sionainn or Sionnain myself (I’ve only ever seen the Anglicised form Shannon, which was a popular name in Ireland in the 1990s/2000s). However, from a Google search, I can see examples of Irish women who have this name, with both spellings.
Irish names can often be spelled in a variety of ways, so I don’t think it’s a case of one being correct and the other being incorrect. The Sionainn spelling does seem to be somewhat more common than Sionnain though… but Shannon is overwhelming more common than both.