Some people say it's a variant of Ciara, but I also read that it's a feminine form of Keir. Does anyone know which version is true?
Probably both! Keir is a common first name in Scotland but is probably originally from the surname Keir/Kerr - for example, iconic Scottish Labour leader Keir Hardie was actually James Keir Hardie, with Keir being his mother's family last name, and many were named after him. Keir is/was sometimes feminised as Keira in Scotland, similar to Lyle > Lyla.
More generally, though, Gaelic-language (both Gaelic from Scotland and Gaelige/Irish) names that start with a C have typically been anglicised with a K (Caol > Kyle, Caoimhín > Kevin, Cellach > Kelly, Ciaran > Kieran, Cillian > Kilian, Ciorstaidh > Kirsty, etc.) The original C- forms are often preferred today in Scotland and Ireland, for authenticity, although the K- forms coexist and some have a legitimate (although more recent) history.
So: Ciara is a feminine form of the old Irish name Ciar/Ciara, meaning black. And in time it became Keara, Keira, Kiara, Kiera, Kira, Kiarra, Kierra, Kerry, Keri, and Kyra in English.
Kira can also be a transliteration from Russian/Cyrillic, and is the feminine form of Kirill/Cyril, meaning young or lord/noble, and also related to Cyrus - all from Greek roots.
I like Kira for the clarity - very easy and straightforward to spell and pronounce. I'd probably go with that or Ciara - but nothing wrong with Keira if you love it.