The most impressive Celtic Christian art was produced from the late 7th to the early 8th century, both in Ireland and in Irish missions in Europe. Manuscripts of books of the Bible were embellished, or "illuminated," with decorative borders and lettering of astonishing intricacy and inventiveness. Complex, twining geometric designs predominated; the rare representations of human faces and figures were abstract and stylized.
The masterpiece of this period is the Book of Kells , which is unsurpassed for the minute perfection of its jewel-like illumination. Other art of the period included large stone crosses carved with interlacing relief decorations; ceremonial religious objects ornamented with gold filigree and colored enamel studs, such as the Ardagh Chalice; and personal ornaments of highly sophisticated design, especially brooches-called pennanular brooches-in gilded bronze and silver.
Much more at the outstandig site http://www.missgien.net/celtic/art.html