By the early 14th century, Ireland had not had a High King since Ruaidri mac Tairrdelbach Ua Conchobair who had been deposed by his son in 1186. Further, the Plantagenet dynasty had been assigned Ireland by Laudabiliter in 1155 and indirectly ruled much of the eastern part of the island. The country was divided between the Irish dynasties that survived the Norman conquest and the Norman-Irish Lordship of Ireland.
In 1258 some of the Gaelic dynasties and clans elected Brian Ua Neill to this position; however he was defeated by the Normans at the battle of Downpatrick in 1260.
The invasion of Ireland:
In 1315, Robert Bruce of Scotland sent his younger brother Edward Bruce to invade Ireland. Bruce's main mission in invading Ireland was to create a second front in the ongoing war against Norman England, draining her of much needed men, materials and finance by creating havoc on the island. This became critical when the Isle of Man was recaptured by Norman-backed Scots from King Robert's control in January 1315, thereby threatening the south and south-west of Scotland and also reopening up a potential source of aid to the Normans from the Anglo-Irish and native Irish.
Added to this was a request for aid from the King of Tír Eógain, Domnall mac Brian Ó Néill. Ó Néill had been troubled by Anglo-Irish incursions to the south-east (the de Verdons), the east (tenants of the Earl of Ulster) and west (also by the Earl of Ulster) of Tír Eógain and in order to retain his lands, he and some twelve of his vassals and allies jointly asked for aid from Scotland. The Bruce brothers agreed, on condition that they would support Edward as King of Ireland, as the brothers envisaged themselves as separate rulers of Scotland and Ireland, while Robert would regain Man and Edward possibly making an attack on Wales, with Welsh support. They personally envisioned "a grand Gaelic alliance against England", between Scotland and Ireland since both countries had a common heritage.
Ó Néill approved of the conditions for himself and on behalf of his vassals, and preparations began. At about this point, Roger Mortimer, 1st Earl of March, received news from Irish sources that an invasion was about to take place, and made his way to Ireland where he held land, mainly in and around the castle and town of Trim. He had previously fought against the Bruces at Bannockburn where he was taken prisoner and freed to return King Edward II's royal seal, lost in the rout.
The Scottish assembly met at Ayr on April 26, 1315, just across the Irish Channel from Antrim. As King Robert did not yet have any legitimate male heir, Edward was proclaimed his legal heir and successor as King of Scotland and all other titles in case of his death. Edward's invasion fleet also mustered there, having received calls to assemble as far back as at least the previous month.
The campaign of 1315:
On May 26, 1315 Edward and his fleet (estimated at in excess of 6,000 men) landed on the Irish coast at points at and between Olderfleet Castle at Larne, and Glendrum. His brother had sailed from Tarbert for the Western Isles with his son-in-law Walter Stewart, to subjugate them till "all the isles, great and small, were brought to his will." Edward meanwhile was swiftly faced by an army led by vassals of the Earl of Ulster such as the de Mandevilles, Savages, Logans and Bissets of the Glens, and their Irish allies, led by Sir Thomas de Mandeville. However they were defeated in battle by the Scots under Thomas, Earl of Moray. Subsequently, the Scots managed to take the town, though not the castle, of Carrickfergus.
In early June Ó Néill and some twelve fellow northern Kings and lords met Edward Bruce at Carrickfergus and swore fealty to him as King of Ireland. The Irish annals state that Bruce "took the hostages and lordship of the whole province of Ulster without opposition and they consented to him being proclaimed King of Ireland and all the Gaels of Ireland agreed to grant him lordship and they called him King of Ireland." In fact, Bruce was never to receive anything more than purely nominal recognition from any of the more powerful Irish Kings, and despite entreatys at various times over the next three years was ignored by those whom he did not directly interest. He did however directly or indirectly rule much of eastern and mid-Ulster.
In late June, Edward proceeded with his army from Carrickfergus along Magh Line (Six Mile Water), burning Rathmore, near Antrim town, which was a holding of the Savages. He then went south by way of the Moiry Pass - called "Innermallan"/"Enderwillane"/Imberdiolan" in contemporary accounts - between Newry and Dundalk. This ancient routeway had been for centuries the passage south out of Ulster into the Kingdom of Mide, Leinster and Munster but because of its narrowness Ulster armies had frequently ambushed and been ambushed at the pass. Here he was met by Mac Duilechain of Clanbrassil and Mac Artain of Iveagh, both of whom had submitted to him at Carrickfergus. Their attempted ambush ended in their defeat and the army pressed on, destroying de Verdon's fortress of Castleroache, and on June 29 attacked Dundalk. The town, another possession of the de Verdon's, was almost totally destroyed with its population, both Anglo-Irish and Gaelic, massacred alike.
In July, two separate armies opposing Bruce met and assemble at Sliabh Breagh, south of Ardee. One was led out of Connacht by Richard Og de Burgh, 2nd Earl of Ulster and his ally, the King of Connacht, Felim mac Aedh Ua Conchobair. The second consisted of forces raised in Munster and Leinster by Justicier, Edmund Butler of Ormonde (father of James, 1st Earl of Ormonde). The Scots-Irish army was located at Inniskeen, ten miles north. In between Sliabh Breagh and Inniskeen was the village of Louth. De Burgh moved his army north of Louth and set up camp while his cousin, William Liath de Burgh attempted to ambush Bruce's forces. While some skirmishing did result in a number of Scots deaths, Bruce refused to give battle and instead, with the Ó Néill, retreated northwards to Coleraine via Armagh. Bruce and Ó Néill sacked and burned Coleraine, threw down the bridge over the river Bann and faced off de Burgh's pursuing army on the opposite bank. While both sides now were experiencing shortages of food and supplies, Bruce and Ó Néill could at least draw support from local lords such as Ó Cathain and Ó Floinn. Mindful of this, de Burgh eventually withdrew back forty miles to Antrim, while Butler had to return to Ormond due to lack of supplies.
In addition to this, Bruce sent separate messages both to King Felim and a rival dynast, Cathal Ua Conchobair, promising to support them if they withdrew. Cathal managed to return to Connacht and had himself proclaimed king, leaving Felim with no choice but to return to put down his rebellion. Worst was to follow: De Burgh found himself deprived of not two but three allies and their armies when his kinsman, Walter mac Walter Cattach Burke deserted back to Connacht at the head of several hundred men, probably to guard his own estates from the upcoming conflict. Thus when in August Bruce and his men crossed the Bann (in four ships supplied by Scots sea captain, Thomas Dun, de Burgh retreated still further to Connor, where on either the first or ninth of September a charge by the Scots-Irish led to his defeat. William Liath was captured and taken as hostage to Scotland by Moray who arrived there on September 15, 1315 to raise more troops, "his ships filled with booty." De Burgh retreated back to Connacht, while other Anglo-Irish took refuge in Carrickfergus Castle.
Finally appraised of the seriousness of the situation, Edward II had on September 1 ordered an assembly of the leading Anglo-Irish, which met at Parliament in Dublin in late October, but no decisive action was taken. On November 13, Bruce marched further south via Dundalk - where, incredibly, some gave them the right hand", i.e., a fight - garrisoned Nobber on the 30th, and advanced to Kells, where he was met by Mortimer. Mortimer had managed to raise a large force consisting both of his Anglo-Irish and Gaelic vassals, in addition to forces of other magnates. At the same time, Bruce was reinforced by Moray who had returned from Scotland with around five hundred fresh troops and supplies. The Battle of Kells was fought on the sixth or seventh of November, with Mortimer being decisively defeated by Bruce. Mortimer was forced to retreat to Dublin while his lieutenant, Walter Cusack, held out at Trim. He almost immediately set sail for England to urge Edward II for reinforcements. At the same time, Governor of Ireland (and Bishop of Ely) John de Hothum began to take drastic action to defend Dublin from Bruce, such as leveling entire tenement and churches.
After sacking and burning Kells, Bruce proceeded to do the same to Granard, Finnea, the Cistercian monastery of Abbeylara and raided Angaile (Annaly), the lordship of Gaelic lord O Hanely. Bruce spent Christmas at de Verdon's manor of Loughsewdy, consuming its supplies entirely and before leaving, razing it to the ground. The only manors left alone belonged to Irish lords intimidated to join him, or that of a junior branch of the de Lacy family who in an effort to gain lands voluntarily joined him.
Defeat in 1318:
After several years of warfare Bruce and his allies did not succeed in holding parts that they had conquered. Famine and disease became widespread, causing him to lose support, and he was defeated at the end of 1318 at the Battle of Faughart.