Irish name nerd here, sorry if I’m too late to help op!
Caolán (pronounced Qway-Lawn or Kay-Lawn depending on which province of Ireland you’re from) is the modern Irish spelling of the name, it means slender lad (Caol meaning slender + án is a diminutive ending for male names) and is strictly male.
Cáelán (pronounced the same as Caolán) was the Middle Irish spelling of the name (we had a spelling reform/standardisation of the language in the early 1900s)
Caelan is the most common Anglicised spelling I’ve seen and is generally pronounced Kay-Lun or Kay-Lin (again regional accent dependent)
As an Irish speaker if I saw Caelan written without accents I would 100% think it’s the Anglicised version of the name and read it as Kay-Lun.
Caolan on the other hand I’d have to ask if it was Kay-Lun (English) or Qway-Lawn (Irish), even if written without the accents, just because sometimes accents get dropped when typing etc.
Caoilfhionn/Caoilinn is pronounced as Qwee-Lyn or Kee-Lyn, it means slender and fair maiden (from Caol again meaning slender + Fionn meaning fair or blonde, when joining the two words they change to the genitive case hence the added i in Caol and the h in Fionn) and is strictly a female name
Because both names look similar and essentially mean the same thing, there are many variations and sometimes they get mixed up with each other. I have met both boys and girls called Caoilfhionn and its variations but only ever boys called Caelan and its variations so far ☺️
Hope this helps!