Usually, in Portuguese, words ending in -ia have the natural stress on the last i, as in "uma livraria". Therefore, if you want to put the stress on the previous syllable, you have to add an accent on its vowel.

"Cecília" AND A paroxytone terminated with rising diphthong "ia". On the other hand "Cecilia" is an eventual proparoxítona. But this separation is pronunciation, some rule like a proparoxítona and those who rule quickly like a proparoxítona. Graphically there is only one correct way: "Cecília".

is a*

Both of them are pronounced exactly the same. 'Cecília' is grammatically correct. 'Cecilia' is grammatically wrong, but names of people do not need to follow grammar rules. So parents when choosing a name to their child can use any spelling they want.