Niall of the Nine Hostages was one of Ireland's most famous historical high kings - however, surprisingly few actual facts about his life are known, and what we have are rather a collection of stories with more of the quality of legend about them. Like the legendary Arthur, from roughly the same time period, Niall is generally agreed to have been a real person, but details of his life are shrouded in the myths that are generated by a culture primarily passed on orally (compare these personages to the reality and detail recorded about their Roman contemporaries, for example). Our written sources are kingly genealogies, legendary tales and various annals and chronicles that often date from centuries after Niall's lifetime.
Broadly speaking, it seems clear that in the late fourth or early fifth centuries of the Common Era, there was a tribal chief called Niall Uí Néill (pronounced, approximately, "Ee Nale") who was later placed on the traditional list of the High Kings of Ireland. The Annals of the Four Masters gives his reign as lasting from 378 CE until 405 CE, when he died, dates which roughly match those of another source text, the Foras Feasa by Geoffrey Keating. However, modern historians calculating backwards from the genealogies of the Uí Néill bloodline (which as we shall later see, is very extensive) place him some 50 years later than this, dying around 450 CE.
Niall's father was Eochaid Mugmedón, the High King of Ireland, and he already had five good sons by his first wife, Mongfind. When he had a sixth by his second wife, Cairenn Chasdub, the daughter of a Saxon king, Mongfind grew jealous and tried to kill this child still in his mother's womb by forcing her to do hard labour. Caireann gave birth to Niall at a well when she was drawing water, but left him there on the ground because she was so afraid of Mongfind. He was discovered by a poet called Torna, who raised him, and when he come of age he returned to Tara to free his mother from her cruel bondage.
When Niall came back and revealed himself, Mongfind grew worried about the rights of her sons, and demanded that the old king name his successor. A series of trials for the brothers were devised by a druid called Sitchenn. First, they were all closed into a burning forge and told to save whatever objects they deemed to be worthy. Niall saved an anvil, Brión a sledgehammer, Fiachra a bellows and a pitcher of beer, Ailill a chest full of arms, and Fergus (clearly the runt of the litter) an armful of wood. For some reason, Niall's anvil was judged to be the greatest item, but Mongfind refused to accept this decision and there was a fresh trial, which turned out to be a mythological classic.
The brothers were all given weapons and sent out hunting, and in their search for water they each came across a well guarded by an ugly old hag. In return for water she demanded a kiss. Fergus and Aillil refused her kiss and returned from the hunt with nothing; Fiachra gave her a chaste peck on the cheek. Only Niall really snogged the face off her, and he was rewarded for his courage when she revealed herself as a beautiful maiden representing the sovereignty of Ireland itself. She named Niall and twenty-six of his descendants as High Kings of Ireland, and in a partially happy ending for Fiachra, she rewarded his peck on the cheek with a High Kingship for two of his descendents. This story of The Loathly Lady recurs in many stories regarding kingship and kingly virtue. I recommend that you read Tlachtga's excellent writeup on the subject.
There are other versions of the story, in which Mongfind also plays a key role - the major one being a story in which Mongfind's brother Crimthann battles with her sons for the kingship, she poisons him (and in the process, herself) and they both die. Following this, the brothers war between each other until only Niall is left alive.
Stories of Niall's death are confusing and widely contradictory, but they all agree on one salient point - he died outside Ireland, at the hand of someone called Eochaid, coincidentally his father's name. Eochaid may have been the son of the king of Leinster, with whom Niall was having a dispute. Niall may have had Eochaid chained to a rock and sent nine warriors to kill him; Eochaid may have freed himself and killed the nine warriors with his chain; eventually Niall is raiding in Britain or in the Alps, and is killed by Eochaid, who may have shot him with an arrow. I'm sorry I can't give you a nice neat end to the nice story, but in Niall's case, history and legend have interwoven to a large degree, and the clearly mythological stories told about his early life are at odds with the (partially) historically-derived tales of his demise.
A Little Reality
A few reality-checks can be made on the above stories. For a start, there was no true High Kingship of Ireland until the 9th century, and even at its best it was a nebulous authority; certainly not a kingship in the same sense as the great European Monarchies. The Irish High King was a ceremonial position designed to bring political unity to a tribal country, and may have been useful in resolving disputes, but really he didn't rule anything but his own tribal area. However, a great deal of effort was made by the new dynasties to legitimise their authority, and this included extending their legitimate genealogy as kings back into the misty past, and linking their names with archetypal figures and well-known stories.
Mongfind probably did not exist as a real person. Not that Niall didn't have a mother, but she was probably just an ordinary, non-Machiavellian lady. Mongfind seems to have been a known supernatural being who would have popped up in various folk tales - an alternative name for Samhain was apparently The Festival of Mongfind, and some people prayed to her at that time.
Niall probably did make raids into Roman Britain (and not the Alps, which may have been a mistranslation of Alba, one of Britain's ancient names). These raids were commonplace at the time, and one of them probably resulted in Saint Patrick being kidnapped and brought to Ireland in the fourth century CE. Some sources state that Niall "carved out an Empire" and took control of large amounts of Britain and some of France; the reality, however, is much more likely to be that his raids were a thorn in the side of the Romans, and when they eventually abandoned Britain for reasons of their own (it was too far away, they didn't like the weather, and they had enough troubles back home) their retreat was hailed as a conquest by various different raiding tribes of the time, the Uí Néills included.
What About The Nine Hostages?
Niall's epithet was Noígíallach (pronounced Nee-gee-ah-lock), which means "Having nine hostages" in old Irish. In the Irish saga The Death of Niall of the Nine Hostages, he was sent hostages from each of the five Irish provinces at the time (Leinster, Ulster, Meath, Connacht and Munster) and also one each from Scotland, Britain, Saxony and the Franks. Other sources differ in the distribution of the hostages, with one stating that they all came from the same region, Airgialla, which literally means "hostage-givers". Hostages were probably given (or taken) for the same reasons as marriages were made between the royal families of conquered or warring territories - as an incentive for those in power to maintain peace, since their rebellion could result in the death of someone dear to them. Although some websites state that Niall was unusually prolific in this respect, it is not clear what their source for this is (other than his name) and the reality is that mutual hostage exchanges would have been a valuable and commonly-used peacekeeping method. It may well be that the most plausible explanation is the one given in relation to Airgialla above - according to T.F. Rahilly, author of Early Irish History and Mythology, an early Irish legal text states that the sole legal responsibility of this named region to the High King was to provide him with nine hostages. This unusual arrangement could have resulted in Niall's name.
The Most Fertile Man In Ireland
Niall got around. It's hard to see how he had time to do all his raiding on Britain and warring with his brothers, considering how much sex he must have had. According to research done in Trinity College, Dublin, Niall is second only to Genghis Khan in his number of descendants. Studying the distribution of Y chromosomes throughout Ireland, the study found that in the area of northwest Ireland that was the stronghold of the Uí Néill clan, 21.5% of people carry a genetic marker that can be traced back to a single person in the fourth or fifth century CE. Moreover, the marker was traceable through exactly the genealogical lines that would be expected to have arisen from the Uí Néills. The following modern Irish names all trace their ancestry back to Niall of the Nine Hostages: O'Neill, Neill, Gallagher, O'Boyle, Boyle, Doherty, O'Doherty, O'Donnell, Connor, Cannon, Bradley, O'Reilly, Flynn, (Mc)Kee, Campbell, Devlin, Donnelly, Egan, Gormley, Hynes, McCaul, McGovern, McLoughlin, McManus, McMenamin, Molloy, O'Kane, O'Rourke and Quinn.
As many as three million men around the world (and possibly as many women) may carry this genetic marker, and therefore are regarded as descendants of Niall. In comparison, Genghis Khan, who conquered almost all of Asia, is reckoned to have approximately 16 million descendants. Irish law in the fourth century permitted divorce and polygyny, concubines were kept by kings, and any and all offspring, legitimate or otherwise, were acknowledged. Not only that, but large numbers of offspring were seen as a marker of power and legitimacy of rule, and therefore powerful men went out of their way to sire as many children as possible. Although it is not recorded exactly how many children Niall had, one of his descendants who died in 1423 is reliably recorded to have had 18 sons and 59 grandsons (the number of daughters and granddaughters was not recorded, since power and succession traditionally passed down the male line).
Stories and Rumors of Niall: http://www.babynamesofireland.com/pages/niall-nine-hostages.html
The Trinity College study: http://www.newscientist.com/article.ns?id=dn8600
A nifty graph of Niall's sexual seeding of Ireland: http://www.nytimes.com/imagepages/2006/01/17/science/20060118_IRELAND_GRAPHIC.html