The following list of books are either rare, expensive, or hard to find books that you should keep an eye out for if you frequent old or used bookshops. This list was compiled by Eachna.

Bone, Antler, Ivory & Horn

by Arthur MacGregor

1985 Croom Helm Ltd in UK, in USA by Barnes & Noble
ISBN 0 7099 32421

A Social History of Ancient Ireland

by P. W. Joyce

1997 Irish Genealogical Foundation, ISBN: 0940134241
This work is in print, but runs about $150 for the hardcover, 2-volume set.
Priced for libraries and schools, not individuals. Covers the whole world
of Irish culture...

(added by Comyn: you can read some of the book online here)

Old Irish and Highland Dress

by H. F. McClintock

(second, enlarged edition)
1950 Dundalgan Press, ISBN unknown

Dress in Ireland

by Mairead Dunlevy

1989 Holmes and Meier in US, ISBN 0-8419-1269-8
This book is still in print, but costs about $60 (hardcover edition only).
The first two chapters are useful for our period

Ancient Danish Textiles from Bogs and Burials

by Margrethe Hald

1980 The National Musuem of Denmark, ISBN 87-480-0312-3
This book is supposedly still available from the National Museum.

Dress in Anglo-Saxon England

by Gale R. Owen-Crocker

1986 Manchester University Press, ISBN 0-7190-1818-8

The Poolbeg Book of Traditional Irish Cooking

by Biddy White

Lennon
Poolbeg Press, ISBN 1-85371-092-X

Land of Milk and Honey: The Story of Traditional Irish Food and Drink

by Bríd Mahon

Poolbeg Press, ISBN 1-85371-142-X

On the manners and customs of the ancient Irish

by Eugene O'Curry

(multiple volume set)
A reprint (from scan) of the original version was released in March, 2010 and is now available at Amazon:
- Comyn has this, and the scans are not that great. A better bet might be to read this online for free at Google Books.

A Guide to Early Irish Law

by Fergus Kelly

The Archaeology of Early Medieval Ireland

By Nancy Edwards

Treasures of Early Irish art, 1500 B.C. to 1500 A.D."

from the collectionsof the National Museum of Ireland, Royal Irish Academy, Trinity College, Dublin / exhibited at the Metropolitan Museum of Art ... [et al.] Published in New York by the Metropolitan Museum of Art [1978, c1977]

Treasures of Ireland : Irish art 3000 B.C.-1500 A.D.

Published in Dublin by the Royal Irish Academy, [c1983]

Irish art in the early Christian period, to 800 A.D.

by Henry, Francoise.

Published in Ithaca, N.Y. by Cornell University Press [1965]