"focloir.ie" give "siblín" -but then doesn't use it in any of the examples! http://www.focloir.ie/en/dictionary/ei/sibling?q=sibling
There is no gender neutral word in Irish; hence this one was invented. Nothing wrong with creating new terms---that's part of what Foras na Gaeilge is for---but when we need to do so, better do it using the existing rich and varied Irish lexicon. What about 'mo chuid fola'? Too street?
Using fuil as a singular countable noun for “sibling” would make “my sibling” m’fhuil. I think that fuil would be too street for me, but I’m old enough that lots of things are too street for me. ;*)
If we’re taking it upon ourselves to coin a replacement for siblín, I’ll suggest comhbhroinn (feminine, second declension; genitive singular comhbhroinne, strong plural comhbhroinnte), as a calque of Ancient Greek ἀδελφός (would this seanghréagachas be a lesser sin than a béarlachas ?). Comhbhroinn is a compound word, so look up its constituent parts in the FGB to find its literal meaning. I suppose that comhbhroinneach could be an analogous adjective (masculine genitive singular comhbhroinnigh, feminine genitive singular comhbhroinní, strong plural comhbhrionneacha), akin to “siblingly”, e.g. Is í Philadelphia an chathair grá chomhbhroinnigh.
Thank you! You're helping me a lot, and it's much appreciated. (I wasn't sure if the accent was used like in Spanish, which I know much better.) Thanks also for explaining about the Munster version; as far as I can gather my Irish ancestors were from Limerick so I think they may have spoken most similarly to that.