Niall Nóigiallach, or Niall of the Nine Hostages, was one of the greatest Irish Kings. He not only ruled Ireland greatly and strongly, but carried the name and the fame, and the power and the fear, of Ireland into all neighbouring nations. He was, moreover, founder of the longest, most important, and most powerful Irish dynasty. Almost without interruption his descendants were Ard Righs of Ireland for 600 years.He was the only son of Eochaid Muigh-medon, High King, and his wife Carthann, daughter of an English (Breton actually under Roman control) King. Eochaid later married Mong-Fionn, daughter of the King of Munster, and had another four sons. These sons were Brian, Fiachra, Ailill, and Fergus.

It is said that Mong-Fionn was jealous of her stepson Niall and wanted her first son, Brian, to succeed Eochaid. She did not rest until she had Niall outcast and his mother Carthann reduced to her servant, carrying water for the Court. But the poet, Torna, who had raised and educated Niall, rescued him from his misery. Then Niall, showing strength of character, even in his early youth, took his mother from her menial task, and restored her to her place. Of Niall's youth there are many legends, but one in particular show the working of his destiny. One day, the five brothers being in the smith's forge when it took fire, they were commanded to run and save what they could. Their father, who was looking on (and who, say some, designedly caused the fire, to test his sons), observed with interest Neill's distinctiveness of character, his good sense and good judgement. While Brian saved the cariots from the fire, Ailill a shield and a sword, Fiachra the old forge trough, and Fergus only a bundle of firewood, Niall carried out the bellows, the sledges, the anvil, and anvil block - saved the soul of the forge, and saved the smith from ruin. Then his father said: "It is Niall who should succeed me as Ard Righ of Eirinn".

During his early reign, Niall consolidated his power at home by subduing his enemies and taking hostages from the royal families in opposition. Meanwhile the Picts were getting wary of the small Irish colony of Dalriada in what is now Scotland. They attacked and severely damaged the colony. In response Niall ventured to Scotia Minor and the land of the Picts here again subduing all and gaining submission through the taking of royal hostages - hence the name Niall of the Nine Hostages. After a short consolidation, he marched south with his Scot and Pict allies against the Romans in Britain. It was there that Niall's forces took hostage one Succat, who is better known by his later name of St. Patrick.

Besides taking St. Patrick in his youth, Niall's reign is known mainly for two things. First, he consolidated the northern region of Ireland and created a dynasty that kept his descendants High Kings of Ireland for 600 years. Second, his military ability led him to the Irish Control of all of Alba and a large part of Britain. He seriously damaged the Romans ability to control Britain and even managed to carve out some Irish controlled territory in France. It is said this Irish Empire was only eclipsed in size by Dathi's, Niall's nephew and successor.

So who were the nine hostages of Naill of Nine Hostages? According to Helen McKimmings book (thanks for the info Ken!), "Niall Noigiallach, of nine hostages is said to have been given his name from having a hostage from each of the five provinces of Erin and four from Britain: Ulster, Connausht, North Leinster, South Leinster, Munster, Dal Riada, Calodonea, Strathclyde, and Northumberia."

References:
Ireland.org
IrishClans.com
Dal Riata from Erin to Alba - Helen McSkimming