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Gaelic Ireland (Irish: Eire Ghaidhealach) was the Gaelic political and social order, and associated culture, that existed in Ireland from the prehistoric era until the early 17th century. Before the Norman invasion of 1169, Gaelic Ireland comprised the whole island. Thereafter, it comprised that part of the country not under foreign dominion at a given time. For most of its history, Gaelic Ireland was a "patchwork" hierarchy of territories ruled by a hierarchy of kings or chiefs, who were elected through tanistry.

Warfare was common in Gaelic Ireland, as territories fought for supremacy against each other and (later) against the Anglo-Normans. Champion warfare is a common theme in Irish mythology. In the Middle Ages all able-bodied men, apart from the learned and the clergy, were eligible for military service on behalf of the king or chief. Throughout the Middle Ages and for some time after, outsiders often wrote that the style of Irish warfare differed greatly from what they deemed to be the norm in Western Europe. The Gaelic Irish preferred hit-and-run raids (the crech), which involved catching the enemy unaware. If this worked they would then seize any valuables (mainly livestock) and potentially valuable hostages, burn the crops, and escape. The cattle raid was often called a Tain Bo in Gaelic literature. Although hit-and-run raiding was the preferred tactic in medieval times, there were also pitched battles. From at least the 11th century, kings maintained small permanent fighting forces known as "troops of the household", who were often given houses and land on the king's mensal land. These were well-equipped professional soldiers made up of infantry and cavalry. By the reign of Brian Boru, Irish kings were taking large armies on campaign over long distances and using naval forces in tandem with land forces.  A typical medieval Irish army included light infantry, heavy infantry and cavalry. The bulk of the army was made-up of light infantry called ceithern (anglicized 'kern'). The ceithern wandered Ireland offering their services for hire and usually wielded swords, skenes (a kind of long knife), short spears, bows and shields. The cavalry was usually made-up of a king or chieftain and his close relatives. They usually rode without saddles but wore armour and iron helmets and wielded swords, skenes and long spears or lances. One kind of Irish cavalry was the hobelar. After the Norman invasion there emerged a kind of heavy infantry called galloglaigh (anglicized 'gallo[w]glass'). They were originally Scottish mercenaries who appeared in the 13th century, but by the 15th century most large tuatha had their own hereditary force of Irish galloglaigh. Some Anglo-Norman lordships also began using galloglaigh in imitation of the Irish. They usually wore mail and iron helmets and wielded sparth axes, claymores, and sometimes spears or lances. The galloglaigh furnished the retreating plunderers with a "moving line of defence from which the horsemen could make short, sharp charges, and behind which they could retreat when pursued". As their armour made them less nimble, they were sometimes planted at strategic spots along the line of retreat. The kern, horsemen and galloglaigh had lightly-armed servants to carry their weapons into battle.  Warriors were sometimes rallied into battle by blowing horns and warpipes. According to Gerald de Barri (in the 12th century), they did not wear armour, as they deemed it burdensome to wear and "brave and honourable" to fight without it. Instead, most ordinary soldiers fought semi-naked and carried only their weapons and a small round shield--Spenser wrote that these shields were covered with leather and painted in bright colours. Kings and chiefs sometimes went into battle wearing helmets adorned with eagle feathers. For ordinary soldiers, their thick hair often served as a helmet, but they sometimes wore simple helmets made from animal hides.
Gaelic Ireland