Slavery may be characterized negatively by an absence of judicial status, meaning that the slave was considered by law to be an object in a slave-owner’s possession rather than as a person in his or her own right. Status as a slave might be temporary or permanent, and although the actual procedures are not known, a slave could obtain status as a free person. There were three sources of recruitment for slaves: (1) prisoners of war, (2) debt slaves, and (3) children born of slaves. Medieval Irish sources for slavery are abundant but often treat the subject cursorily and non-systematically. A full picture of Irish slavery must therefore remain impressionistic in character.
The most common names for slaves in Irish were mug for male and cumal for female slaves. Cumal was also widely used as a unit of value for cattle and land. "Martyrologies" often refer to slave labor as an image of personal debasement. The vita of St. Senan tells of the men of Corcu Baiscind who were admonished to obey St. Senan to not suffer such hunger that "a man would sell his son and daughter in distant territories for nourishment." A vita of the ninth century relates that St. Ciaran, a slave to the king, had to grind the grain every day. Slaves are never associated with husbandry but mainly with heavy agricultural labor such as sowing, harrowing, thrashing, and grinding.
Ship raids on Britain in the fifth century after the collapse of the Roman Empire provided prisoners of wars who were treated as slaves. These raids seem to have ceased as a result of the stabilization of Britain in the seventh century. Children born of these prisoners continued to be a source of an Irish slave population, although they are rarely mentioned in the idealistic status system depicted by early medieval Irish laws. Recurring mention of the sale of children in hunger years attests to the existence of "debat-slavery" as an institution throughout the early Middle Ages.
The effect of the Viking attacks and subsequent settlements was to accentuate slavery as a social institution.
Viking warfare did not respect the sanctity of monasteries and brought about a change in the norms of warfare, which included an acceptance to reduce prisoners of war to slave status. The Irish annals record 23 instances when Vikings took prisoners en masse, which must be taken as an indication of slaving operations. While hostages for tribute were termed geill, the annals refer to these prisoners as brat (captives). The early instances of Viking slave raids do not, however, indicate large-scale operations for a full-blown slave market, but rather seem to be spectacular acts of defiance and humiliation against the enemy. After the battle of Tara in 980, the king of Meath is reputed to have freed all the Irish slaves of Dublin, an act that was repeated by Brian Boru’s and Mael Sechnaill’s joint action on Dublin after the victory over Leinster and the mercenaries of Dublin in 999. During the first half of the tenth century, slaves were still a by-product of a particular kind of war, namely retaliatory actions and military adventures designed to vaunt the capabilities of the would-be successor. Overall, there is no evidence to suggest that the institution of slavery in Ireland and even in Dublin was anything more than a marginal phenomenon of luxury for the nobles.
However, in connection with the heavy expenditures caused by the struggle for the kingship of Ireland in the early eleventh century, Irish kings began taking captives in large numbers. In the decisive campaigns up to 1014, punitive actions seem to have greatly increased. The Cenel nEogain king Flaithbertach was the leading slaver in a number of actions on neighboring territories. In 1011, he united with the son of Brian Boru and allegedly took many cows and 300 captives (brait, a word hitherto restricted to Viking assaults) from the Cenel Conaill, and the following year he is credited with the largest booty any king had taken of captives and kine from the Ulaid. In later years, the annals repeatedly note the massive taking of prisoners, and more mundane events were changed by new attitudes to the defeated. Irish warfare had traditionally seen many plain raids that were not part of a larger political scheme but rather must be seen as seasonal traditional manifestations of the bravado of young warriors. This long-established custom was called crech, a prey or a raid for cattle. By the mid-eleventh century, the taking of captives also became part of these heroics. The rising power of the Northern over kings is marked in the annals by heavy exactions upon neighboring kingdoms. The Ua Conchobair kings of Connaught, who were at times near achieving total supremacy over Ireland, also practiced the new kind of warfare in their campaigns. The climax came in 1109, when Muirchert-ach of the Dal Cais mustered a large force against the Ui Briuin of Connaught and took many captives from the islands of Loch Oughter. The Ui Briuin took revenge upon Meath, the ally of Muirchertach, by burning, killing, and leading off many captives. The final blows to Dal Cais supremacy were accompanied by great predatory expeditions in 1115 and 1116, but the prisoners of the last campaign were released afterward as an homage to God and to St. Flannan of Killaloe—the patron saint of the Dal Cais.
What were the driving motives behind the massive taking of prisoners by Irish kings of the eleventh and early twelfth centuries? First, there was a striking similarity between the warfare of Dublin and Irish kings. Simple lessons of the humiliating function of massive imprisonment to the prestige of any king were well learned by the Irish, and it seems plausible that once learned, they put it to their own use. Further, Irish cattle raids and petty warfare between minor kings took on a far larger and more devastating character when the Irish invited Viking warriors for wars of conquest and paid them in kind by the wealth of the enemy, including prisoners of war. We know that the great struggles of the over kings for supremacy were largely decided by the use of naval fleets. These fleets were either indirectly controlled by the over king as a consequence of their control of Norse cities or they were hired from Norse settlements in Ireland or the Scottish Isles. The decline of Dublin’s political power forced many warriors either to settle or to take up freebooting, more or less out of control of the Dublin king. These half-independent warriors may have supplied the Dublin slave market with captives that had not been taken because of political complications, but simply for profit. From the middle of the twelfth century, we know that the Dublin fleet was hired for thousands of cattle that were driven to the city in payment. It is also conceivable that payment in the eleventh century was in slaves.
As the evidence stands, there is, however, no way to substantiate the hypothesis that the Irish captives of war were in fact sold to Norse slave dealers. What exists is a relatively clear-cut case that slavery became more widespread during the course of the eleventh century. We have much circumstantial evidence of the importance of slavery to Irish kings in eleventh century writings such as Lebor na Cert (Book of Rights) and Cogadh Gaedhel re Gallaibh. The distinction in these texts between male and female slaves reveals some functions of slavery. Female slaves are referred to as "full-grown," "swarthy," "fair," "graceful," and "valuable;" and the Leinster king is obliged to give "eight women whom he has not dishonored." Male slaves are described as "lads," "hard working," "strong-fisted," "willing," "expensive," and "spirited." If we may deduce anything from these descriptions, the slaves seem primarily to have been intended for the household: as servants, concubines, mountebanks, and the drabants of the court. The old use of cumal for a female slave was evidently obsolete by 1100, and instead mna (daera) or the crude banmog were used. In the Leinster list, Dublin is entitled to "thirty women with large families"—an indication perhaps of the furnishing of Dublin warriors with concubines. Further, Lebor na Cert draws a clear distinction between native and foreign slaves ("foreigners who do not know Irish," "women from over the great sea"), an indication that not only were slaves recruited by internal warfare but some were also supplied by foreign trade.
Ireland has no mineral wealth, and foreign luxury goods could be bought by Irish kings mainly for two export goods, cattle and people. Labor and concubines were in demand wherever a new elite had established itself, and hides for parchment were in strong demand. Tenth and eleventh century wars and not least the Norman conquest of Britain must have generated a strong market for the Irish commodities. Very little is known about the actual trade mechanisms and balances, but one indicator is the growing number of instances recorded in the annals of the taking of slaves by the Irish. In the eleventh century, Dublin was probably the prime slave market of western Europe, furnishing customers in the British Isles, Anglo-Saxon as well as Norse, and the Scandinavian countries. In 1102, however, Dublin’s slave trade to Bristol was prohibited on religious grounds, while the trade also seems to have been despised for its antisocial character. Demand in Scandinavia declined for the same religious and social reasons as it did in Britain. The trade and Irish slave raids seem therefore to have petered out in the early twelfth century. However, some trade must have continued, as indeed the Irish synod of 1170 welcomed the Norman Conquest as just punishment for the abuses of the slave trade. Slavery as such was not put to an end overnight, as we are well reminded by the synod of Armagh of 1170. Even as late as 1235, the mark of slavery was still felt by some people; in Waterford a man was known as Philippus Ley sing, Philip the manumitted, or freed slave.