The Irish language gallóglach is derived from gall "foreign" and óglach; from Old Irish oac (meaning "youth") and Old Irish lóeg (meaning "calf" but later becoming a word for a "hero"). The Old Irish language plural gallóglaigh is literally "foreign young warriors".(The modern Irish plural is galloglagh.)

According to the Oxford English Dictionary (OED), the English language singular, gallowglass, is an anglicisation of Old Irish plural gallóglaigh, ^[4] rather than the singular. The OED, which prefers the spelling "galloglass", provides examples of various English plural forms, dating back to a c. 1515 use of "galloglasseis". The OED states that the conventional English plural, gallo(w)glasses, is "probably the result" of the duplicate, redundant plural suffix -es being added to the Irish plural.^[4] (The OED notes that while the modern Irish plural galloglagh appears to be "etymologically correct", it is recorded later than English gallowglasses.) For instance, Shakespeare uses the form "gallowglasses" in Macbeth.^[1] The OED adds that "galloglasses is found ... in our earliest quot.", even though "in some early instances", a relatively early, alternate English plural form, galloglas, is also used.^[4] Hence, English gallowglass is probably a singularised form, which post-dates the plural, gallowglasses.

The gallowglass were from the western coast of Scotland, principally Argyll and the Western Isles.^[5] Their weapons were swords and axes. Each was usually accompanied by a man to see to his weapons and armour and a boy to carry provisions.

A description from 1600 speaks of the gallowglass as "pycked and seelected men of great and mightie bodies, crewell without compassion. The greatest force of the battell consisteth in them, chosinge rather to dye then to yeelde, so that when yt cometh to handy blowes they are quickly slayne or win the feilde".^[6][7]

The first record of gallowglass service was in 1259, when Aedh Ó Conchobair, King of Connacht, received a dowry of 160 Scottish warriors from the daughter of Dubhghall mac Ruaidhri, the King of the Hebrides. They were organised into groups known as a corrughadh, which consisted of about 100 men.

The importation of gallowglasses into Ireland was a major factor in containing the Anglo-Norman invasion of the 12th century, as their ranks stiffened the resistance of the Irish lordships. Throughout the Middle Ages in Ireland, gallowglass troops were maintained by Gaelic Irish and Hiberno-Norman lords alike. Even the English Lord Deputy of Ireland usually kept a company of them in his service.

In return for military service, gallowglass contingents were given land and settled in Irish lordships, where they were entitled to receive supplies from the local population.

By 1512, there were reported to be fifty-nine groups throughout the country under the control of the Irish nobility. Though initially they were mercenaries, over time they settled and their ranks became filled with both Scots-Norse and many native Irish men.

In 1569, Turlough Luineach O'Neill (the O'Neill) married Lady Agnes Campbell, daughter of Colin Campbell, 3rd Earl of Argyll, and widow of James MacDonald, 6th of Dunnyveg. Her dowry consisted of at least 1,200 gallowglass fighters. Along with two young men as support and friends on top to assist or fight this could easily have numbered over 5,000 current and future gallowglasses coming into the area.^[8]

They were noted for wielding the massive two-handed sparth axe (a custom noted by Geraldus Cambrensis, died c. 1223, to have derived from their Norse heritage) and broadsword or claymore (claidheamh mór). For armour, the gallowglass wore a mail shirt over a padded jacket and an iron helmet; he was usually accompanied by two boys (like a knight's squires), one of whom carried his throwing spears while the other carried his provisions.

Shakespeare mentions gallowglasses in Macbeth, although along with other aspects of the play it is an anachronism, as the historical Macbeth lived in the 11th century:

The merciless Macdonwald,
Worthy to be a rebel, for to that
The multiplying villainies of nature
Do swarm upon him, from the Western isles
Of kerns and gallowglasses is supplied

In the paper "A Description of the Power of Irishmen", written early in the 16th century, the Irish forces of Leinster are numbered at 522 horses and five battalions of gallowglass (gallóglaigh) and 1,432 kerne, and those of the other provinces were in like proportion. Mac Cárthaigh Mór commanded 40 horses, two battalions of gallowglass, and 2,000 kerne; the Earl of Desmond 400 horses, three battalions of gallowglass, and 3,000 kerne, besides a battalion of crossbowmen and gunners, the smaller chieftains supplying each their quota of men.

In 1517, "when the reformacion of the countrye was taken in hand", it was reported that the Irish forces in Thomond were 750 horses, 2,324 kerne, and six "batayles" of gallowglass, the latter including 60 to 80 footmen harnessed with spears; each of these had a man to bear his harness, some of whom themselves carried spears or bows.

Every kerne had a bow, a "skieve" or quiver, three spears, a sword, and a skene or sgian (Irish scian or Scottish Gaelic sgian), each two of them having a lad to carry their weapons. The horsemen had two horses apiece, some three, the second bearing the "knave" or his attendant.

The 16th century in Ireland saw an escalation in military conflict, caused by the Tudor conquest of Ireland. Gallowglass fighters were joined by native Irish mercenaries called buanadha (literally "quartered men") and by newer Scottish mercenaries known as "redshanks". During the First Desmond Rebellion, Lord President of Munster Sir William Drury ordered the execution of 700 captured gallowglasses.^{[citation needed]}

Despite the increased use of firearms in Irish warfare, gallowglasses remained an important part of Hugh Ó Neill's forces in the Nine Years' War. After the combined Irish defeat at the Battle of Kinsale in 1601, recruitment of gallowglasses waned, although Scottish Highland mercenaries continued to come to Ireland until the 1640s (notably Alasdair Mac Colla). They fought under the Irish general Owen Roe O'Neill at the Battle of Benburb when O'Neill had an overwhelming victory in 1646. The gallowglasses of the Mac Cárthaigh Riabhaigh are recorded as having attacked Mallow in County Cork as late as 1645.^{[citation needed]}

Images of gallowglasses fighting as mercenaries in European mainland armies were sketched by Dürer in 1521 and later by French and Dutch artists.^[9] Gallowglasses served in the Dutch Blue Guards, Swiss Guard, the French Scottish Guard, and the forces of King Gustavus Adolphus of Sweden in his invasion of Livonia during the Thirty Years' War.^{[citation needed]}

Millford in County Donegal, historically called Ballynagalloglagh (from Irish: Baile na nGallóglach), is a small town and townland whose Irish name means "town of the gallowglasses".^[10] A battle between the Irish (helped by gallóglaigh) and the English took place on a hill in the townland and this is where the name comes from.

• Úlfhéðnar
• Fianna
• Gallogly, a surname

• G. A. Hayes McCoy, Irish Battles, Appletree Press, Belfast, 1990.
• Colm Lennon, Sixteenth Century Ireland: The Incomplete Conquest, Gill & MacMillan, Dublin 1994.
• The Galloglass Project (compiled at TCD, placed online at UCC)