About Ancient Celtic art :

• Celtic art is associated with the peoples known as Celts; those who spoke the Celtic languages in Europe from pre-history through to the modern period, as well as the art of ancient peoples whose language is uncertain, but have cultural and stylistic similarities with speakers of Celtic languages.
• Celtic art is a difficult term to define, covering a huge expanse of time, geography and cultures.
• A case has been made for artistic continuity in Europe from the Bronze Age, and indeed the preceding Neolithic age; however archaeologists generally use “Celtic” to refer to the culture of the European Iron Age from around 1000 BC onwards, until the conquest by the Roman Empire of most of the territory concerned, and art historians typically begin to talk about “Celtic art” only from the La Tène period (broadly 5th to 1st centuries BC) onwards.
• Early Celtic art is another term used for this period, stretching in Britain to about 150 AD. 
• The Early Medieval art of Britain and Ireland, which produced the Book of Kells and other masterpieces, and is what “Celtic art” evokes for much of the general public in the English-speaking world, is called Insular art in art history. This is the best-known part, but not the whole of, the Celtic art of the Early Middle Ages, which also includes the Pictish art of Scotland.
• Typically, Celtic art is ornamental, avoiding straight lines and only occasionally using symmetry, without the imitation of nature central to the classical tradition, often involving complex symbolism.
• Celtic art has used a variety of styles and has shown influences from other cultures in their knotwork, spirals, key patterns, lettering, zoomorphics, plant forms and human figures.
• As the archaeologist Catherine Johns put it: “Common to Celtic art over a wide chronological and geographical span is an exquisite sense of balance in the layout and development of patterns.
• Curvilinear forms are set out so that positive and negative, filled areas and spaces form a harmonious whole.
• Control and restraint were exercised in the use of surface texturing and relief. Very complex curvilinear patterns were designed to cover precisely the most awkward and irregularly shaped surfaces”.

Source: Wiki