'Celtic' can be pronounced with both a hard and soft 'c' - both are correct. The hard 'c' is probably more correct etymologically, since the word probably derives from Latin and Greek, whereas the soft c follows the convention in English that 'c' followed by 'e' is usually soft.

However, in the UK at least, for some considerable time the convention has been to use the hard 'c', except when talking about the Scottish football team.