These were originally Scots mercenaries, but by the 16th Century their clans or "septs" had often been settled in Ireland for two or three centuries (the most famous were the MacDonalds and the McSweeneys); they were still mercenaries, but often owed loyalty to a particular noble (in fact in 1568 there were three septs of the "Queen Majesty’s Galloglasses"). They usually wore an iron bascinet, and either a mail shirt or a short cape of mail over a padded quilted coat called a "cotun" and their characteristic weapon was a heavy two-handed axe, up to six feet long, which could chop the enemy’s head off with a single blow (it was still used in 1588 when McLaghlin M’Cabb killed 80 Spaniards from the Armada with one).

Gallowglasses were organized in "battles" of 80 or 87 men, but each gallowglass was accompanied by two boys, who carried his supplies, armour, and his secondary weapons, three light Irish javelins or "darts".

(Source: http://www.MyArmoury.com)

"The galloglas were largely a spent force by the end of the sixteenth century; Thomas Gainsford, writing in 1618, could say that ‘the name of galloglass is in a manner extinct’…"

"… They were not entirely a closed body, for although their leaders and most of their men were still of Scottish ancestry, some Irishmen were admitted to their ranks. But their real place was in a narrower kind of warfare, the warfare of earlier times. They were incapable of rapid expansion to meet the unprecedented call for manpower of the great struggle that had now begun. Although some of them, like O’Neill’s own galloglas, the MacDonald Sept of Antrim had become ‘shot & pikemen’ by 1595, the galloglas on the whole had not done much to adapt themselves to the changed conditions of fighting which followed the extension of the use of firearms. O’Neill could not look to the galloglas for much support in this war, even though it was a war waged in defense of the institutional system whereby they lived. Nor as it happened, could he hope for much help from the Scots. He tried hard, with promises of pay and plunder, to attract mercenaries from the Highlands and bands of MacDonalds, MacLeans and Campbells did , from time to time, serve him and his allies; but things were changing in Scotland too. The power of the Campbells of Argyll was growing and they were setting the other clans against one another.The disturbed conditions which they found in Scotland increased the difficulties of O’Neill’s recruiters, who had also to contend with the efforts of Queen Elizabeth’s agents in Scotland to stop the mercenary trade at its source. King James VI, although he slyly encouraged O’Neill, would never help him; to do so might endanger his chances of succession to the English throne."

"… O’Neill, O’Donnell and the rest of the lords could not have fought as they did if they were dependent on the kinds of fighting men whom their ancestors had used. but they were not so dependent. Denied the assistance that they required from the ranks of galloglas and the Scots mercenaries, bodies that were the one old-fashioned and the other erratic in its comings and goings, they had to make up their numbers elsewhere. They did so by raising large forces of bonnaughts in their own territories, forces that were, in effect, the Irish counterpart of the national militias which, in the same age, superceded the condittieri and the landsknecht and the Swiss and the other mercenaries all over Europe."

(Source: Quoted directly from Irish Battle: A Military History of Ireland by G.A. Hayes-McCoy)