3.5 SRD This site is a MUST see. Under the Open Gaming License, several DnD fans have cranked out some spectacular compilations of the 3.5 source documents. (As someone who has already spent $100+ on the three core books, I was very happy to find this gem on the web.)  

ScreenMonkey Play DnD and other pen and paper RPGs online with this nifty tool. NBOS also makes Fractal Mapper which is relatively easy to use cartography software that produces high quality maps.

BabyNamesof Ireland is a great site to give inspiration for character and NPC names.

Irish Literature, Mythology, Folklore and Drama: One of the best and most extensive collection of links I've seen. Some excellent links to translations of old Irish tales, and more info, mythical and historical, than you can shake a fey stick at!

Stone Pages: As best described on their site: "Stone circles, dolmens, standing stones, cairns, barrows and hill forts: welcome to the first and most comprehensive online guide to European megaliths and other ancient sites." From Ireland to Italy! Wonderful for a GM to use during sessions to illustrate the land of Eyru.

Internet Medieval Sourcebook: An excellent source of links to all things Medieval. Well indexed and up to date links.

History of Medieval Towns: This is a wonderful site that details the history of two medieval towns. Everything from early maps, town legal code and even the town's seal are included here. A great source for a GM planning a medieval town and the inspiration behind the Eyrian city of Lynn.

The Arador Armour Library: A dictionary of terms, construction techniques, pictures, and even a forum to share ideas about medieval armour and weaponry. Wonderful source for GMs and players who want to add realism to their campaign.

Heraldry on the Internet: Just what it says! While Eyru is modeled after a historical period that greatly pre-dates heraldry, this site's information is too useful to pass up.

Historic Roots: A wonderful page containing a database of clans and a very brief history of the use of the tartan. Excellent for GM's and PC's looking for family names to use in game.

The Grey Labyrinth: While not historically related, this site is perfect for keeping players on their toes. A good riddle or a mind-bending puzzle is all but irresistible to many people of Eyru. This collection of puzzles is sure to give the GM something to challenge the players with aside from bands of angry giants and ill-tempered beasts.

The World of Celtic Art: A collection of ancient and modern Celtic art links with some useful historical information on Celtic culture as well.

Online Gaelic Dictionaries: Searchable Gaelic dictionaries! Many place names and proper names in Eyru have been taken from (or in part, derived from) Gaelic dialects. Irish Gaelic is most prominent in these proper names though there is a smattering of Manx and Scottish.

Open Gaming Foundation: A link to information on the OGF which provides a copy of the OGL which Wizard of the Coast's d20 system is published under.

Brigantia: Iron Age Celtic Re-Enactment: A historical re-enactment group that is serious about its work. A good deal of research has gone into their roles and much of their findings are posted on the site.

Beginner's guide to Irish Gaelic Pronunciation: Not sure how to pronounce the name's of some of Eyru's important figures and characters? (That makes two of us! :) This guide will help since most of the linguistics of the campaign are based on Irish Gaelic.

Celtic Art and Cultures: This is a fabulous collection of images. Inspirational for GMs and players who want to add some authenticity to their game.