Celtic tribes originated in Central Europe and share similar languages, religious beliefs, traditions and cultures. It is believed that Celtic culture began to develop as early as 1200 BC. Their heritage is most prominent in Ireland and the United Kingdom, and their language and cultural traces are still prominent today.
Celtics Origin
By the third century BC, the Celts controlled most of the European continent north of the Alps, including what is now Ireland and Britain.
It was on these islands on the western coast of Europe that Celtic culture survived and developed with the expansion of the Roman Empire on the European continent.The Celtic race never really existed as we understand it today. They are loose mixtures of tribes, communities, and different groups that come together to achieve common purposes, such as defense, worship, trade, and hunting. Unlike the classical civilizations of Greece and Rome, the Celtics left almost nothing.
Celtic religion
When Christianity arrived in Ireland with St. Patrick in 432 AD, many Celtic traditions were incorporated into the "new" religion. According to some historians, after the massive killing of the Druids, the religious leader of the Gaelic tribe, Catholicism could replace the island as the main religion. However, even though Christianity has a new reputation, traces of Celtic culture still exist. The national symbol of Ireland, the clover, represents the "Trinity" of the Catholic tradition, namely Father (God), Son (Jesus Christ) and Holy Spirit.
Language
In Wales, Celts call it Cymru (Cymru), the mother tongue is Welsh, and it is still widely spoken in the region. In Scotland, although a few people still speak Celtic (Scots Gaelic), the local branch of the British Broadcasting Corporation (BBC) is called BBC Alba, the Celtic name of the area.
Above is a Celtic Knot pendant. 
Celtic influence
When we see the degree of Celtic influence in Western European culture, we can understand that the Greeks and Romans were puzzled by the original origin of the culture. The impact is not only linguistic, but also social, technical, spiritual, educational, artistic and cultural.
The westward movement seems to be mainly caused by imitations triggered by infiltration (settlement and trade). When the Celtics entered the Western lands, it appeared to be two-way traffic, which was due to the remains of the megalithic culture (Neolithic and Bronze Age) along the Atlantic coast in the religious and language region.