So this has come up on a few of my groups, and I thought since I did one on Irish Language resources I would do one for online resources to reliable sites to Celtic mythology.
Celtic Digital Initiative: (from their website) The aim of the Celtic Digital Initiative (CDI) is to make scarce resources available in an electronic format to students and scholars, both within UCC and beyond.
Mary Jones: (From their website) This is an attempt to collect as many possible early and medieval texts produced in the "Celtic" countries, or on Celtic themes (hence the inclusion of Continental Arthurian works). Some works are actually links off-site; others are provided here at the CLC.
Corpus of Electronic Texts (CELT): (From their website) To bring the wealth of Irish literary and historical culture (in Irish, Latin, Anglo-Norman French, and English) to the Internet in a rigorously scholarly and user-friendly project for the widest possible range of readers and researchers. CELT (the Corpus of Electronic Texts) caters for academic scholars, teachers, students, and the general public, all over the world.
A Corpus of Ancient Written References to the Druids: Note I am only recommending this page of the website as I have not read the rest of it. (From the website) First of all, these are primary sources, not secondary sources. In other words, these ancient records are to a large extent the basis of our knowledge about the Druids. This means that by becoming familiar with them and understanding them, one can gain a more direct and firmer familiarity with the subject matter than by merely consulting modern works, which are, after all, derived from them. Often one can demonstrate by referring to them that a statement regarding the Druids in a modern work is false or unwarranted. On the other hand, this also means that these sources are not critical or scientific works - the Greeks, Romans and Church Fathers who authored them were not impartial and usually did not have good information about the Druids. Therefore, not everything they wrote about them was necessarily true. These are texts to be analyzed, not simply believed at face value. This is an important point, so I will say it again in bold: These are texts to be analyzed, not simply believed at face value.
Sacred Texts: I don't think this site needs an introduction.
Alexei Kondratiev's Celtic Story Telling: A series of six Youtube videos, where Alexei talks about Celtic Story telling.