I can't answer all of your questions but I can talk a little bit about the gallowglasses in the Irish context. Contrary to popular belief, gallowglasses or gallóglaigh (lit. 'foreign warriors') were a high-late medieval staple of Irish warfare and did not exist before the 13th century (thanks, Shakespeare!). The fact that they were considered gall -foreigners- indicates that they were not perceived as having any significant ties with Irish culture.

Irish warfare during the early medieval period seems to have revolved around small and mobile bands of lightly armed and armoured warrior-aristocrats who traveled to raids and battles on horseback. Warfare seems to have been focused on cattle raiding and denying the enemy aristocracy's means of sustenance by killing their peasant clients and laying waste to farmland, rather than seeking pitched battles. It was a very mobile and fast-paced kind of warfare which prized agility and dexterity over brute strength and physical endurance; with the exception of iron helmets and wooden shields, body armour was almost nonexistent in pre-viking Ireland.

The arrival of the Normans in the 12th century upset this style of warfare with their introduction of heavy cavalry and the widespread use of chain maille body armour. The Normans are actually frequently called 'grey foreigners' in Middle Irish sources, because they were perceived as being clad head to toe in grey, metal armour. The Irish had no real answer to Anglo-Norman warfare until the advent of the gallowglasses in the 13th century. This isn't to say that the Normans were some unstoppable martial force; on the contrary, their heavy cavalry was mostly useless in Ireland's boggy, hilly and thickly forested landscape. Eventually the Hiberno-Norman aristocracy synthesized Irish and Norman military practices, forms of military vassalage and retinue hosting.

Gallowglasses can be seen as a development in reaction to these changes in Irish military practice. They would be raised from Norse-Scottish populations in the Hebrides and south-western Scotland where they served in the retinues of Scottish aristocrats. So how did they end up in Ireland? Initially gallowglasses were attained by Irish warrior-aristocrats by marriage: Ireland and Scotland still belonged to the same cultural continuum during this period, and Irish lords would frequently marry Scottish noblewomen. Retinues of gallowglasses were a common and expected dowry that would be provided by the Scottish father-in-law. By the 14th century, many gallowglass extended family groups settled in Ireland due to the social dislocation caused by the wars of independence.

Gallowglasses quickly became a staple of Irish warfare; though first employed by the native Irish, they were also employed by the Hiberno-Normans. They were massive men armoured with a knee-length maille shirt, iron helmet and armed with a massive two-handed axe or sword. The gallowglasses provided the Irish aristocracy with a leveling mechanism in the face of English and Hiberno-Norman forces.

By the Tudor reconquest, gallowglasses formed the backbone of Irish military resistance to the English state, and many seem to have formed mercenary bands for hire which existed alongside formal retinues held by lords. They had a pretty nasty reputation among English commentators like Edmund Spenser, who wrote that:

from the tyme that they enter into that coorse, they doe use all the beastlie behavior that may bee to oppresse all men: they spoile aswell the subjecte as the enemye; they steale, they are cruell and bloodye, full of revenge, and delighte in deadly execucon, licensious, swearers, and blasphemers, comon ravishers of weomen, and murtherers of children.

Spenser's work is basically propaganda meant to convince the Queen's court of the need to subjugate and improve Ireland, but it nevertheless reflects commonly held assumptions about the Irish and Irish soldiers.

The gallowglasses died out in Ireland alongside the independence of the Irish aristocracy, as following the 9 Years' War, Queen Elizabeth forbade the aristocracy in Ireland to hold retinues of soldiers, and at the same time forbade the existence of 'masterless men' under the penalty of death - these were bands of mercenary warriors and soldiers that had thrived under the militaristic atmosphere of the Tudor conquest. If gallowglasses wanted to continue their trade (and most of these guys would have been raised since childhood to be warriors; they wouldn't have had much experience with farming or crafts anyways) they had to take to the continent, where many of them served as mercenaries in the armies of continental monarchs.