THE CELTIC Cross didn't always have christain conotations they were also used for way markers or signs to important pagan sites. I think they are lovely however you choose to use them in your art

It sounds to me like it was religious but not used as grave markers until later. This is what Wikipedia says:

In Ireland, it is a popular myth that the Celtic Christian cross was introduced by Saint Patrick or possibly Saint Declan during his time converting the pagan Irish. It is believed that Saint Patrick combined the symbol of Christianity with the sun cross, 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.[3]

In Celtic regions of Ireland and later in Great Britain, many free-standing upright crosses (or high crosses) were erected by Irish monks, 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, on the island of Iona and in the Hebrides, as well as the many in Ireland.[4] Other stone crosses are found in the former Northumbria and Scotland, and further south in England, where they merge with the similar Anglo-Saxon cross making tradition, in the Ruthwell Cross for example.

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. The Celtic cross is often made of stone. After the 15th century, ringed high crosses ceased to be created in the Celtic lands, other than a few obscure examples.

The Celtic Revival of the mid-19th century led to an increased use and creation of Celtic crosses in Ireland. In 1853 casts of several historical high crosses were exhibited to interested crowds at the Dublin Industrial Exhibition. In 1857, Henry O'Neill published Illustrations of the Most Interesting of the Sculptured Crosses of Ancient Ireland. These two events stimulated interest in the Christian and non-Christian Celtic crosses as a symbol for a renewed sense of heritage within Ireland.

New versions of the high cross were designed as fashionable cemetery monuments in Victorian Dublin in the 1860s. From Dublin the revival spread to the rest of the country and beyond. Since the Celtic Revival, the ringed cross became an emblem of Celtic identity, in addition to its more traditional religious symbolism.

I aggree with the abuve but also celtic crosses as their known as now, have been in lots of civalivations
good luck
Alex

Many monasteries and churches in Ireland, Scotland and the North of England had tall celtic crosses stood outside, which were not necessarily tomb markers. It may have been a throw-back to the celtic and pictish tradition of erecting freestanding monolits. Some transitional carvings including pagan symbols and celtic crosses survive in Scotland on standing stones. These stones were cultural markers and spiritual rallying points.

Celtic crosses have been used more recently for war memorials and the like, not necessarily graves.