From Wikipedia:

A Celtic cross is a symbol that combines the cross with a ring surrounding the intersection.

It is the characteristic symbol of Celtic Christianity, though the symbol has older, pre-Christian origins. Such crosses formed a major part of Celtic art. This design is also referred to as the Irish Cross, or as the Cross of Iona.

In Celtic regions of Ireland and Great Britain many free-standing upright crosses – or high crosses – were erected, beginning at least as early as the 7th Century. Some of these 'Celtic' crosses bear inscriptions in runes. There are surviving free-standing crosses in Cornwall (famously St Piran's cross at Perranporth) and Wales, in the island of Iona and in the Hebrides, as well as the many in Ireland. Other stone crosses are found in Cumbria and the Scottish Borders, however some of these are of the similar Anglo-Saxon cross making tradition. The most famous standing crosses are the Cross of Kells, County Meath, Ireland, Ardboe Auld Cross, Ardboe, County Tyrone, Northern Ireland, the crosses at Monasterboice, County Louth, Ireland, and the Cross of the Scriptures, Clonmacnoise, Ireland

There are numerous representation of crosses combined with a circle, even before Christianity. Often called "sun cross", they can be found in Bronze Age Europe (Nordic Bronze Age, Urnfield culture).

The archaic English word for cross as an instrument of torture is rood (literally "pole", cognate with rod). The word cross in English derives only indirectly from Latin crux via Old Irish and possibly Old Norse, introduced in the 10th century.

Irish Myth
In Ireland, it is a popular myth that the celtic cross was introduced to the island by Saint Patrick during his time converting the pagan Irish. It is believed that he combined the symbol of Christianity, a cross, with the symbol of the sun, to give pagan followers an idea of the importance of the cross by linking it with the idea of the life-giving properties of the sun.[citation needed]

Campus Ministry
The Presbyterian Church (USA) has two campus ministries that operate under the name Celtic Cross. The name is significant to the Presbyterian Church's Scottish-Irish heritage, and the symbol of the Celtic Cross is often viewed as a symbol of witness and evangalism among many presbyterians.[1][2]

The cultural associations of the Celtic cross, and the sun cross from which it is derived, have been co-opted by white nationalist and neo-fascist groups since the 1960s. This is because of its connotations of Christianity, Western culture, and old European traditions. The symbol can also sometimes be identified with radical nationalists of a Third Positionist or Catholic nationalist persuasion.

Far right supporters usually use a basic design which is made up of simple lines, without any of the ornamental complexity of traditional Celtic crosses. In these recent adaptations, it is sometimes also called a sun wheel.

This new political connotation has almost eclipsed the traditional meaning of the symbol in France, Italy and many other European countries. In France, the symbol was adopted by the groups Occident and the Groupe Union Droit. In Italy, the symbol has been banned from being shown within stadiums, as it is considered a sign of fascism and racism.

Celtic crosses are also associated with political movements advocating greater independence or other measures with respect to Celtic minorities, such as Breton nationalism.