The Medieval period

Christian/Celtic Europe

"With the arrival of St. Patrick in the 5th century CE, full-scale conversion to Christianity took place, and monasteries became the principal artistic centers."
http://www.missgien.net/celtic/art.html

Folio 114 recto from the Book of Kells; the arrest of Christ.

http://www.eccentricbliss.com/tag/book-of-kells/

Folio 188 recto from the Book of Kells; the beginning of the Gospel according to St. Luke.

http://www.eccentricbliss.com/tag/book-of-kells/

celtic art was very much influenced by the Christian religion, this can be clearly seen by these two examples just here. The first illustration is of Christ. Very particular characteristics of celtic art can be seen in this illustration in particular. Rich colours such as blues, reds and golds are very often used in celtic art. Another characteristic's which is well known in celtic art is intertwining shapes, like celtic knots, almost like plant vines. lastly, intertwining animals. 

In christ's hair you can see this typical intertwining style of celtic art, at the top of the illustration, the intertwining animals can be seen. 

The below image being again from the book of Kells, but a beginning page to a part of the gospel, just goes to show the importance of this religion. Imagine the time and effort would would have to take to create a piece as highly detailed as this. To myself the painting almost reminds me of royalty. The rich celtic golds, blues and reds can be seen in this illustration also. 

"Manuscripts of books of the Bible were embellished, or "illuminated," with decorative borders and lettering" http://www.missgien.net/celtic/art.html

Folio 34 recto from the Book of Kells; Chi-Rho page; introducing St. Matthew’s account of the Nativity of Our Lord Jesus Christ.

Again this exquisit style is used to present the begging of a chapter in this religious book. As these examples show typical examples of celtic art, it becomes clear that celtic art would not of been what it was without the influence of Christianity. 

The celtic era came around the ending to the fall of the Roman Empire. This is therefore when the country became Christian, after the fall of not only the Roman Empire, but obviously the Roman Catholic faith. 

Here is another example of Celtic Christan art from another book other than the book of Kells. Again you can see the extremely similar style in this books illustration.

http://www.britannica.com/EBchecked/media/4224/Interlace-patterns-of-the-initial-page-of-the-Gospel-According?topicId=341885

"Medieval manuscripts were usually produced by a team of scribes and illustrators. However, the entire Lindisfarne Gospels is the work of one man, giving it a particularly coherent sense of design. According to a note added at the end of the manuscript less than a century after its making, that artist was a monk called Eadfrith, Bishop of Lindisfarne between 698 and 721." http://www.bl.uk/onlinegallery/sacredtexts/lindisfarne.html