In 891 Donal, son of Hugh Finnlaith, succeeded as King of Ulster on the death of Flaherty, who fell in a skirmish with the men of Clanbrassil, the Mc'Canns, in the present Co. Armagh. Donal held the kingship for some time, until his right was disputed by his brother, Niall Glundubh. It was then agreed to settle the question by battle, which was prevented by the two being elected to reign jointly. There is no details as to the arrangements adopted.
Donal resigned in 903, probably influenced by the death of his son and heir, Flan, who died the previous year. Donal died in the year 911, according to the Annalists, after a well spent life.
Donal was senior to Niall, who however was of a more bolder and more enterprising disposition; qualities which counted for everything at this time, Niall was given the name Glundubh, which translates Black Knee.
In 907 one Cearnaghan Mac Dulgan, perpetrated sacrilegious violence in the Cathedral of Armagh, from which he took a certain captive who had fled there for refuge, and drowned him in Lough Cirr near the City.
The Archbishop complained to Niall of this for in Ireland as in other Christian countries many of the churches had the right of sanctuary.
Niall on learning of the outrage committed by Cearnaghan, had him seized and drowned in the same Lough Cirr as punishment for the crime.
In 913 Niall, who acted as heir apparent to Flan, invaded West Connaught, and gave an overthrow to the men of Tír Awley and Ua Mhaill, who lost great numbers.
In the same year he was obliged to march against the Ulidians, who were not inclined to acknowledge his sway. At Glarryford, near the Ravel Water, in Antrim, he defeated Loingseach, Chief of Dal Araidhe, and marching southwards, encountered and vanquished Hugh, son of Eochagan, Prince of Ulidia, who signed a treaty of peace at Tullahoge in Cinel Eoghan in November.
In 914 Niall suppressed a rising against Flan Sinna by his sons Donogh and Conor, from whom he exacted pledges and hostages.
Soon after this, Flan died peacefully at Kinneigh, a religous house in Kildare, in the 68th year of his age.
Niall ascended to the throne and in the same year he renewed the great Fair of Tailtin, where the laws were promulgated; a military review was held; and a court for the hearing of appeals.
He next prepared to take the field against the Norsemen, who, after a rest of nearly half a century, were again ravaging Ireland.
At this time they had a great fleet and fortress at Waterford, from which, under Attar ( with the Danes of Brittany ) and Raghnall and Sitric, sons of Godfrey, King of Northumberland, they wasted the whole of Munster.
Against these Niall marched at the head of his troops, and encamped at Tobar Glethrach, in the neighbouthood of Clonmel, on the 22nd of August, 915, On the same day the Danes under Sitric advanced to meet him, when after an indecisive engagement both armies separated and retired to their camps. Hearing that reinforcements under Raghnall were on their way from Waterford, Niall went out of his camp with a small force against them and narrowly escaped being cut off.
On the morrow he again advanced against the combined forces, whom he at lenght succeeded in defeating and driving to their ships after a sanguinary engagement in which 1100 men fell on both sides, but the greater number on the side of the Danes.
The next year Niall again advanced on Dublin, when, again deceived as to their strength of the Danes, and confident of victory, he roared these words to his army, according to the four Masters.
Whoever wishes for a speckled boss
and a sword of sore inflicting wounds
a green javelin for wounding wretches
let him come early in the morning to Ath Cliath ( Dublin ).
The Danes, confident in their numbers, came forth to meet the Irish, when a bloody battle was fought at Kilmashoge, near Rathfarnham, where Niall suffered a disastrous defeat, being left on the field with Conor O' Melaghlin. his heir apparent; Flaherty, son of Donal, heir of Ulster; Hugh, Prince of Ulidia, Maelmithidh, son of Flanagan, Lord of Breagh; the Lord of Oirghilla, and others.
In lamentation of Niall was written the lines:
Sorrow this day is noble Ireland
Without a valiant Chief of hostage reign
It is to see the heaven without a sun
To see Magh Néill without Niall.
Niall's tomb, made of transverse and upright unhewn blocks of stone, is still to be seen on the hill overlooking the battlefield in the grounds of Glen Southwell, near St. Columba's College.
A Cairn on a mountain's summit, a many a kernes mound,
had thrice its height of tribute stones, and thrice its paces round.
Nor ever a branch of Ogham, nor Christain scribe to tell,
The lofty and generous heart that sleeps in its silent cell.
Niall by his Queen Gormfhlaith had three sons, (1) Conaing ( Royal Heir of Ireland ), Who killed 1,200 Ulidians in battle near Lough Erne. He died 937, leaving a son Domnall, whose son, Fergal ( King of Ailech 980 to 989 ) he died 1001, and whose descendants were local kings of Tulach Og until 1068.
(2) Muircheartach Na Cochall Craicenn" of the Leather Cloaks" ( King of Ailech, Royal heir of Ireland )