When established Christianity came to Ireland, it was forced to adapt to the realities of Irish society and patterns settlement. Although initially organized along episcopal lines, Ireland's lack of any major centers of population (most people lived in isolated farmsteads dotted across the landscape; villages don't even appear in indigenous Irish territory until well after the arrival of the Normans) and constantly shifting political hierarchies favoured a more dispersed and less bureaucratic approach to Church structures.

The monastic form dominated early Irish Christianity and can be seen as an "Irish solution to an Irish problem", to quote Daibhi O Croinin who described this adaptation as a "tribal church". Consequently, the Irish monastic church more or less resembled the Irish tuath - the "petty-kingdom" which was the most basic unit of the Irish political hierarchy, being ruled by hereditary abbots (mostly drawn from the same aristocratic lineages as the ruling aristocracy) who in fact held the equivalent legal status to the king of a tuath.

Besides resembling Irish kingdoms and kings in structure and legality, Irish monasteries and their abbots were also great landowners and held clients just as a secular king would. The system of monastic allegiance can even be said to resemble the interregional hierarchies of great provincial kings - the major monastic centers of Armagh, Clonmacnoise and Iona established sattelite communities across Ireland (and in the latter's case, Britain), and jostled to become the principal ecclesiastical authority in Ireland.

Irish society was also thoroughly militarized - low-intensity warfare in the form of cattle raiding and pillaging was a fact of life, while major interregional wars of supremacy begin to be fought in the 8th and 9th centuries. Irish monasteries and churches were not immune from this, and were frequently pillaged and burned by Irish lords - between 612 and 792, Irish annals record the destruction by burning of thirty churches, and only three appear to be attributed to natural disasters. Ecclesiastical sites are mentioned as principal objects of destruction in full-out Irish warfare, and if the annals are to be believed, the Irish pillaged more Irish monasteries during the Viking age than the Vikings themselves!

So the Irish monastery was organized like an Irish kingdom and ruled by a hereditary abbot who not only had the legal status of a king, but was overwhelmingly drawn from the warrior-aristocracy (Cormac mac Cuilennáin is a perfect example - he was a bishop who ascended to the kingship of the province of Munster in 901, holding both titles simultaneously while waging far-reaching wars into Leinster, destroying many ecclesiastical sites). It should not be surprising then, that given the context of early medieval Ireland's militarized society, and the jostling for monastic supremacy mentioned above, Irish monasteries sometimes took to the battlefield like secular kingdoms.

Donnchadh Ó Corráin lists several of these engagements listed in various annals between the 7th and 8th centuries: a battle takes place at the community of Birr in 664; in 760 Birr meets the monastic center of Clonmacnois on the battlefield; in 764 Clonmacnoise fights Durrow, in which 200 members of the latter's community are slain; in 783 the abbot and bursar of Ferns go to war against one another in some sort of civil war; in 807 Cork and Clonfert meet in a pitched battle where "an innumerable slaughter of the ecclesiastical men and superiors of Cork" is the result; in 817 the King of Osraige (I believe) and the monastery of Taghamon fight the community of Ferns and 400 people are slain, with the King apparently becoming the vice-abbot of Ferns as a result. Besides intra-monastic warfare, it appears that some abbots waged wars exclusively against secular kings.

Not only did Irish monasteries resemble and act like secular kingdoms, but as the only major centers of population on the entire island until the establishment of Scandinavian settlements, they appear to have had the manpower to fight devastating wars against one another. Secular Irish warfare was likely fought by small bands of mounted warrior-aristocratic clients of chief aristocrats, who were in turn the clients of greater aristocrats on the political hierarchy - these monastic communities on the other hand appear to have grown economically, demographically and politically powerful enough to muster significant military forces that, as Ó Corráin notes, some of their engagements are referred to in source material as bellum (war), while the same source describes fights between several principal regional kingdoms as belliolum (little battle).

While not a direct answer, an indicator that at least some monks knew which end of the sword goes into the enemy is that the earliest fencing manual that we know of - the [I.33](http://wiktenauer.com/wiki/Walpurgis_Fechtbuch_(MS_I.33)) was likely written by a monk, and depicts monks utilising swords and bucklers.

Whilst monks were generally prohibited from committing violence, there were some monastic orders that were established for the specific purpose of fighting. These soldier-monks fought in the crusades and participated in conflicts across the Middle East, Iberia, North Africa, Mediterranean, and the Baltic, from the twlefth century until 1798. The Knights Templar were one such order, a group of monk-knights originally intended to militarily defend pilgrims in the Crusader States in Palestine. The Templars followed a monastic rule and swore the three vows of poverty, chastity, and obedience like the brethren of any non-military monastic order. Other religious orders originally founded to have a military role include the Knights of Santiago, Order of Calatrava, and the Sword Brethren, to name a few.

There were also monastic orders that began as non-military but then later militarised. The Hospital of St John of Jerusalem (also known as the Knights Hospitaller), the Order of St Lazarus, and the Teutonic Order, were all originally hospitaller orders founded in the Crusader States and operating charitable, palliative, and medical facilities. Some time after their foundation, each order militarised out of necessity, becoming military religious orders in the style of the Templars, as the Crusader States were in need of manpower and a military order could act as a small standing army.

The best general, though dated, overview of the military orders is Desmond Seward's The Monks of War (London, 1972). David Marcombe, Leper Knights: The Order of St Lazarus of Jerusalem in England, c.1150-1544 is one of the few English accounts of the Lazarites (most scholarship on this order is in French). Helen Nicholson, The Knights Hospitaller (Woodbridge, 2001) is a good overview of the Hospitallers. For the early history of the Teutonic Order, see Nicholas Morton, The Teutonic Knights in the Holy Land, 1190-1291 (Woodbridge, 2009). Malcolm Barber's The New Knighthood (Cambridge, 1994) is still one of the main accounts of the Templars.