The Celtic [pronounced Keltik] is a fun language. By which I mean it'd confusing. The letter 'C' is the easiest though, it's pronounced 'k' so Cillian is pronounced 'killian' a Ceran as 'key-ran'. It gets far more complicated when you get into words like Lughnasdh or brideo'gas.
Celtic comes from the race of people that were Celts. Pronounced Keltic. The Scottish football team uses the Irish "Erse" pronunciation "seltic"; because they are "Irish Catholic", as opposed to the Glasgow Rangers, Protestant.