Marking a cross on a piece of stone is an act which generally speaking is not datable. Mostly all we can do is to indicate the occasions on which this might be done.
Crosses were used to mark the boundary of a church or monastic site. This might be quite simply stones with a cross marked on them. Sometimes we find them still in the place where they were first put, as for example at the Reefert Church Glendalough.
Crosses were sometimes marked on ancient stones: Clear Island, Co Cork, was pagan cult-pillar, Legananny Co Down, Breastagh Co Mayo, Giantsgrave Co Tipperary, Doonfeeny, Co Mayo were ancient standing stones. This may have indicated that, in the eyes of the inscriber, Christianity had taken over.
Crosses were used to indicate graves, such as at Kilfountain on Dingle peninsula.
Crosses were also placed to help guide pilgrims on their way across country, such as pilgrims going to Iniskeel in Co Donegal.
Crosses also feature as an integral part of the ‘turas’, that is doing the rounds at holy sites and places of pilgrimage.
Crosses were also placed in or on top of outdoor altars (leachta) where the church was small or at tomb-shrines or sites of pilgrimage.
Some crosses are scratched on stone by hundreds of pilgrims.
Crosses come in all sorts of sizes and all kinds of designs, Greek ones, Latin ones, Maltese ones and so on. Immense creativity is evident in the designs in which twirls and symbols abound.
There are about ten crosses in Ireland which have the chi-rho design. This was the sign associated with Constantine the Great. Drumaqueran Co Antrim and Kilshannig on the Dingle Peninsula, Co Kerry are good examples.
Crosses were used in processions. Some of the symbols used on them may have been transferred to stone crosses. For example the ‘little churches’ on top of crosses may be symbols of the ‘Anastasis’ (‘Resurrection’) inside the Church of the Holy Sepulchre in Jerusalem and thus symbols of the Resurrection on the Crosses in question.
Many crosses were of course made in wood. Some were sheathed in leather, held in by metal bosses. These seem to lay behind the striking designs of the Ahenny and Kilkieran crosses in Co Kilkenny
The most famous crosses are the crosses displaying scenes from scripture, built no doubt for prestige, but also serving the many pilgrims who flocked to the great monasteries. The great examples are Clonmacnoise, Monasterboice, Durrow etc. These were part of an open sacred or liturgical area in front of great churches, which space also included round towers.
There are some good books available to provide details of these crosses. See:
P Harbison, Irish High Crosses, Drogheda 1994;
Elinor D U Powell, The High Crosses of Ireland, Liffey Press 2007;
Hilary Richardson & John Scarry, An Introduction to Irish High Crosses, Mercier Press 1990.