Old Irish language in the context of Middle Irish

The Norse–Gaels (Old Irish: Gall-Goídil; Irish: Gall-Ghaeil; Scottish Gaelic: Gall-Gàidheal; Manx: Goal-Gael, 'foreigner-Gaels') were a people of mixed Gaelic and Norse ancestry and culture. They emerged in the Viking Age, when Vikings who settled in Ireland and in Scotland became Gaelicised and intermarried with Gaels. The Norse–Gaels dominated much of the Irish Sea and Scottish Sea regions from the 9th to 12th centuries. They founded the Kingdom of the Isles (which included the Hebrides and the Isle of Man), the Kingdom of Dublin, the Lordship of Galloway (which is named after them), and briefly ruled the Kingdom of York (939–944 AD). The most powerful Norse–Gaelic dynasty were the Uí Ímair or Ivar dynasty.

Over time, the Norse–Gaels became ever more Gaelicised and disappeared as a distinct group. However, they left a lasting influence, especially in the Isle of Man and Outer Hebrides, where most placenames are of Norse–Gaelic origin. Several Scottish clans have Norse–Gaelic roots, such as Clan MacDonald, Clan MacDougall and Clan MacLeod. The elite mercenary warriors known as the gallowglass (gallóglaigh) emerged from these Norse–Gaelic clans and became an important part of Irish warfare. The Viking longship also influenced the Gaelic birlinn and longa fada, which were used extensively until the 17th century. Norse–Gaelic surnames survive today and include MacIvor, MacAskill, and [Mac]Cotter.

Norman Irish or Hiberno-Normans (Irish: Normánach; Old Irish: Gall 'foreigners') is a modern term for the descendants of Norman settlers who arrived during the Anglo-Norman invasion of Ireland in the 12th century. Most came from England and Wales. They are distinguished from the native Gaelic Irish, although some Normans eventually became Gaelicised. The Hiberno-Normans were a feudal aristocracy and merchant oligarchy which controlled the Lordship of Ireland. The Hiberno-Normans were associated with the Gregorian Reform of the Catholic Church in Ireland and contributed to the emergence of a Hiberno-English dialect.

Some of the most prominent Hiberno-Norman families were the Burkes (de Burghs), Butlers, and FitzGeralds. One of the most common Irish surnames, Walsh, derives from Welsh Normans who arrived in Ireland as part of this group. Some Norman families were said to have become "more Irish than the Irish themselves" by merging culturally and intermarrying with the Gaels.

Fortriu (Latin: Verturiones; Old Irish: *Foirtrinn; Old English: Wærteras; Pictish: *Uerteru) was a Pictish kingdom recorded between the 4th and 10th centuries. It was traditionally believed to be located in and around Strathearn in central Scotland, but is more likely to have been based in the north, in the Moray and Easter Ross area. Fortriu is a term used by historians as it is not known what name its people used to refer to their polity. Historians also sometimes use the name synonymously with Pictland in general.

The Dagda (Old Irish: In Dagda [ˈd̪aɣða], Irish: An Daghdha) is considered the great god of Irish mythology. He is the chief god of the Tuatha Dé Danann, with the Dagda portrayed as a father-figure, king, and druid. He is associated with fertility, agriculture, masculinity and strength, as well as magic, druidry and wisdom. He can control life and death (cf. his staff), the weather and crops, as well as time and the seasons.

He is often described as a large bearded man or giant wearing a hooded cloak. He owns a magic staff (lorc) of dual nature: it kills with one end and brings to life with the other. He also owns a cauldron (the coire ansic) which never runs empty, and a magic harp (Uaithne, though this may be the name of the harper), which will not play unless called by its two bynames, and the harp can fly itself to the Dagda when thus beckoned. He is said to dwell in Brú na Bóinne (Newgrange). Other places associated with or named after him include Uisneach, Grianan of Aileach, Lough Neagh and Knock Iveagh. The Dagda is said to be the husband of the Morrígan and lover of Boann. His children include Aengus, Brigit, Bodb Derg, Cermait, Aed, and Midir.

Meath (/miːð/ MEEDH; Modern Irish: Mí; Old Irish: Mide [ˈmʲiðʲe]) was a kingdom in Ireland from the 1st to the 12th century AD. Its name means "middle," denoting its location in the middle of the island.

At its greatest extent, it included all of County Meath (which takes its name from the kingdom), all of County Westmeath, and parts of counties Cavan, Dublin, Kildare, Longford, Louth and Offaly.

The Kingdom of East Breifne or Breifne O'Reilly (Old Irish: Muintir-Maelmordha; Irish: Bréifne Uí Raghallaigh, IPA: [ˈbʲɾʲeːfʲnʲə iː ˈɾˠəil̪ˠiː]) was an historic kingdom of Ireland roughly corresponding to County Cavan that existed from 1256 to 1607. It took its present boundaries in 1579 when East Breifne was renamed Cavan, after Cavan town, and shired into Ulster. Originally part of the older Kingdom of Breifne, East Breifne came into existence following a protracted war between the ruling O'Rourke clan and the ascendant O'Reillys which culminated in the division of the kingdom in 1256. The Kingdom was ruled by the dynasty of the Ó Raghallaigh (O'Reilly) and lasted until the early 17th century.
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The Fomorians or Fomori (Old Irish: Fomóire, Modern Irish: Fomhóraigh / Fomóraigh) are a supernatural race in Irish mythology, who are often portrayed as hostile and monstrous beings. Originally they were said to come from under the sea or the earth. Later, they were portrayed as sea raiders and giants. They are enemies of Ireland's first settlers and opponents of the Tuatha Dé Danann, the other supernatural race in Irish mythology; although some members of the two races have offspring. The Tuath Dé defeat the Fomorians in the Battle of Mag Tuired. This has been likened to other Indo-European myths of a war between gods, such as the Æsir and Vanir in Norse mythology, the Olympians and Titans in Greek mythology, and the Devas and Asuras in Indian mythology.

One theory is that the Fomorians were supernatural beings representing the wild or destructive powers of nature; personifications of chaos, darkness, death, blight and drought.

Halfdan Ragnarsson (Old Norse: Hálfdan; Old English: Halfdene or Healfdene; Old Irish: Albann; died 877), nicknamed Hvitserk (“White-Shirt”), was a Viking leader and a commander of the Great Heathen Army that invaded the Anglo-Saxon kingdoms of England starting in 865.

Halfdan was one of six sons of Ragnar Lodbrok named in Norse sagas; his brothers and half-brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye, Ubba and Hvitserk. Because Halfdan is not mentioned in any source that mentions Hvitserk, some scholars have suggested that they are the same individual – a possibility reinforced by the fact that Halfdan was a relatively common name among Vikings and Hvitserk "white shirt" may have been an epithet or nickname that distinguished Halfdan from other men by the same name.

Olaf Guthfrithson or Anlaf Guthfrithson (Old Norse: Óláfr Guðrøðsson [ˈoːˌlɑːvz̠ ˈɡuðˌrøðsˌson]; Old English: Ánláf; Old Irish: Amlaíb mac Gofraid; died 941) was a Hiberno-Scandinavian (Irish-Viking) leader who ruled Dublin and Viking Northumbria in the 10th century. He was the son of Gofraid ua Ímair and great-grandson of Ímar, making him one of the Uí Ímair. Olaf succeeded his father as King of Dublin in 934 and succeeded in establishing dominance over the Vikings of Limerick when he captured their king, Amlaíb Cenncairech, in 937. That same year he allied with Constantine II of Scotland in an attempt to reclaim the Kingdom of Northumbria which his father had ruled briefly in 927. The forces of Olaf and Constantine were defeated by the English led by Æthelstan at the Battle of Brunanburh in 937.

Olaf returned to Ireland in 938 but after Æthelstan's death the following year Olaf left for York where he was quickly able to establish himself as king, with his brother Blácaire mac Gofraid being left to rule in Dublin. Olaf and Æthelstan's successor Edmund met in 939 at Leicester where they came to an agreement regarding the division of England between them. This agreement proved short-lived, however, and within a few years Vikings had occupied the Five Boroughs of Derby, Leicester, Lincoln, Nottingham and Stamford. Olaf died in 941 and was succeeded in Northumbria by his cousin Olaf Cuaran. At the time of his death, the Irish annals title him "king of Danes" and "king of the Fair Foreigners and the Dark Foreigners".

Navan Fort (Old Irish: Emain Macha [ˈeβ̃ənʲ ˈβ̃axa]; Modern Irish: Eamhain Mhacha [ˌəunʲ ˈwaxə]) is an ancient ceremonial monument near Armagh, Northern Ireland. According to tradition it was one of the great royal sites of pre-Christian Gaelic Ireland and the capital of the Ulaidh. It is a large circular hilltop enclosure—marked by a bank and ditch—inside which is a circular mound and the remains of a ring barrow. Archeological investigations show that there were once buildings on the site, including a huge roundhouse-like structure that has been likened to a temple. In a ritual act, this timber structure was filled with stones, deliberately burnt down and then covered with earth to create the mound which stands today. It is believed that Navan was a pagan ceremonial site and was regarded as a sacred space. It features prominently in Irish mythology, especially in the tales of the Ulster Cycle. According to the Oxford Dictionary of Celtic Mythology, "the [Eamhain Mhacha] of myth and legend is a far grander and mysterious place than archeological excavation supports".

Navan Fort is the heart of the larger 'Navan complex', which also includes the ancient sites of Haughey's Fort (an earlier hilltop enclosure), the King's Stables (a manmade ritual pool) and Loughnashade (a natural lake that has yielded votive offerings).

Mac Giolla Phádraig (Irish: [mˠək ɟɪl̪ˠə fˠaːd̪ˠɾˠəɟ]; Old Irish: Mac Gilla Pátraic) is a native Irish dynastic surname which translates into English as "Son of the Devotee of (St.) Patrick". In the medieval period, the Mac Giolla Phádraigs were hereditary kings of Osraige; today, the anglicised version of the name is commonly "Fitzpatrick".

Lir or Ler (meaning "Sea" in Old Irish; Ler and Lir are the nominative and genitive forms, respectively) is a sea god in Irish mythology. His name suggests that he is a personification of the sea, rather than a distinct deity. He is named Allód in early genealogies, and corresponds to the Llŷr of Welsh mythology. Lir is chiefly an ancestor figure, and is the father of the god Manannán mac Lir, who appears frequently in medieval Irish literature. Lir appears as the eponymous king in the tale The Children of Lir.