Who were the Celts?
Concise background on Celts

Celtic Art and Cultures
EXCELLENT art history site w/ lots of pictures of artefacts, timelines, essays etc.

C.EL.T  (Corpus of ELectronic Texts)
brings the wealth of Irish literary and historical culture to the Internet, for the use and benefit of everyone worldwide. It has a searchable online textbase consisting of 940 contemporary and historical documents from many areas, including literature and the other arts.

Celtic Encyclopedia
Compiled and edited by Knud Mariboe. Good basic resource, well referenced.

Folklore Archives  
a comprehensive archive of Celtic folklore as well as folklore and stories from around the world.

Clannada  
Good all around site including languages, history, folklore, art, and great links.

Celtic Studies Resources
From an Opinionated Digital Medievalist

Celtic Studies Bibliography
from the Celtic Studies Association of North America (CSANA)

Celtic Artist List   
a list of links to various Celtic artists involved in a Celtic Art Yahoo group  hosted by Steven Walker.

E-KELTOI
On-line journal edited by Bettina Arnold. Multi-disciplinary

INSIGHT
On-line journal edited by Stoifan MacAmhalghaidh. New perspectives in Irish Studies

IRQUAS 
The Irish Quaternary Studies Online Project - a series of highly scholarly lists run by the self-proclaimed 'dangerous amateur' Stiofan MacAmhalghaidh
The lists cover every aspect of Irish Studies, and often include the top scholars in the fields of ARCHAEOLOGY: Stone Age, Bronze Age, Iron Age, Early Medieval, Late Gaelic, Early Modern and Settlement Studies; ARTS & DESIGN: Architecture, Art History and Literature; GEOGRAPHY: Cartography, Geology, Place Name Studies and Settlement Studies; HISTORY: Early Medieval, Late Gaelic, Early Modern, Economic History and Social History; and SCIENCES: Agriculture, Education, Linguistics and Natural History. The site also includes Hillfort and Ringfort Databases. 

Bookfinder is an excellent source for books (out of print, rare or new) as is the Advanced Book Exchange where you can store books that you are looking for, and they will inform you when one becomes available.