Waterford, Demetia, Dyfed

Irish raiders were causing the British widespread problems throughout the 4th and 5th centuries, and to combat this, The homeless Deisi tribe were settled in Demetia by Magnus Maximus. Existing evidence confirms this by suggesting the Roman authorities asked for the help of Aed Brosc in keeping Irish pirates away from the western coast of Britain. When the last British king died without a male heir, the Déisi were on hand to fill the breach.

The Irish Deisi (or Déssi) tribe of the County Waterford region in Ireland, were settled by the existing Roman authorities in the Demetian region by 382. Their leader, Aed Brosc, was one of his people's key figures. His grandfather, Eochaid Allmuir, had been forced to leave his homeland after a bid for independence failed, and was severely crushed by the Irish High King. The descendants of Aed’s eldest son, Urb, founded the Kingdom of Brycheiniog between 420-450, while his younger son, Triffyn Farfog, married the heiress of the Demetian Kings. Like most Irish Royalty, Eochaid claimed descent from Beli Mawr, the Celtic Sun-God, through his son, Miled. The name DYFED (also known as Dyfor) originates from one of the former Roman civitates within the kingdom, and seemed to be more popular for the new Deisi rulers than Demetia. The early Deisi settlers were very receptive to the bonuses of the Roman way of life, and gave their children Romano-British names. These were recorded by the Early Welsh in Welsh format, and later rulers became completely Welsh in background as the Roman influence was severed. the Déisi rulers of Dyfed initially used the Roman title of "Protector" (protictoris) instead of the British equivalent of king. This practice continued at least as far as Vortipor, whose memorial stone was discovered at Castell Dwyran.

The Viking settlement of Waterford city was founded in the 9th and 10th centuries. In the twelfth century, the chieftains of the Decii assumed the surname O'Feolain. Viking fleets were sailing on the Suir from the ninth century and from the era of Brian Boru, the Irish high-kings would hire Waterford ships captains to run their navy. The building of those ships represented an enormous community effort: it took 2,650 man days to build each of the great warships. In 1051 King Diarmait MacMurchada (Murphy) of Leinster had abducted Derbforgaill, wife of King Tigern n Ruairc (O'Rourke) of Breifne (the northern parts of modern Leitrim and Cavan. Although she was restored to Tigern n the next year, the insult was never forgiven and in 1166 O'Rourke allied himself with the new high-king, Toirrdelbach Conchobhair (O'Connor), and drove Diarmait out of his kingdom. Diarmait, taking his daughter Aoife with him, sailed to Bristol and thence to France to seek aid from Henry. The king responded with a general letter to his liegemen authorizing any who wished to ally themselves with Diarmait to do so. The baron who took up the challenge, and Aoife's hand in marriage, was a Norman whose grandfather Gerald had married a Welsh princess: he was Richard de Clare, Earl of Pembroke, known as Strongbow. In 1169, his uncle, Maurice Fitzgerald, helped Diarmait capture Wexford, and in August 1170 Strongbow himself arrived. `It was,' lamented the writer of The Annals of Ulster, `the beginning of the woes of Ireland'. Strongbow and his fellow adventurers, using the advanced Norman military techniques which had brought victory throughout Europe, conquered Leinster and part of Meath, and took Waterford and Dublin. The O'Reilly chief of the sept was known as Breffny O'Reilly. Throughout history the size of their territory ebbed and flowed depending on the relative power of their rivals the O'Rourkes. In 1171 Lorcán O'Tuathail (Laurence O'Toole), Archbishop of Dublin, strongly resisted the Anglo-Norman invaders; indeed his sept was particularly successful in its opposition, aided in large part by the mountain terrain which it inhabited.

County Waterford was the ancient home of Deisi Mumhan, Magh Femin, Uí Liathain and Coscraidh. The Viking settlement of Waterford city was founded in the 9th and 10th centuries. By the 13th century territorial names included the Decies, Gal-tir, Hy Fodhladha, and Uactar Tire.

The earliest inhabitants of this portion of the island were a tribe designated by Ptolemy Menapii, who occupied also the present county of Wexford. Prior to the seventh century, mention is made of two small tracts, one called Coscradia, and the other Hy-Lyathain, on the south, about Ardmore; but these designations appear to have merged at an early period in that of Decies, given by the preponderating power of a tribe called the Desii, or Decii, who occupied the central and larger portions of the county at the time of the English invasion. They are said to have been originally planted in Meath, and gave name to the barony of Deece. In a contest for the chieftaincy of that tribe in the middle third century, a large number was compelled to abandon that territory, and to remove southwards, and they ultimately settled themselves in a tract of country extending from Carrick-on-Suir to Dungarvan, and thence eastward to Waterford harbor. From this point Decie in Meath, and Decie in Munster, were called respectively North and South Decie; the latter also bore the Irish name Nan-Decie. But Aengus Mac Nafrach, King of Munster, in the fifth century, enlarged the territories of the Decii by annexing to them the lands of Magh Femin, comprising the present barony of Middlethird, and the large extended plains near Cashel, called Gowlin, together with the country about Clonmel: and from this period the designation of Decie-Thuasgeart, or North Decie, became applied only to his grant; the former territories in Waterford still retaining the distinctive appellation of Decie-Deisgeart, or South Decie. In the ninth century, the population of this territory was augmented by the Danes, who, under a leader named Sitric, conquered and retained the maritime district bordering on the harbour of Waterford, then nearly insulated, and forming the present barony of Gaultier, "the land of the Gauls, or Foreigners." The Danes founded the city of Waterford, and made it their chief station. In the twelfth century, the chieftains of the Decii assumed the surname O'Feolain.

The Déisi Mumhan were of Co. Waterford and southern County Tipperary. Septs included Ua Bric (O'Brick), Ua Faeláin (Phelan/Whelan). The earlier origins of the Déisi are stated to be on the plains of the River Boyne, where the tribes of the Déisi Brega continued to flourish in early medieval times. An ancient genealogy has the Ua Fáeláin of Déisi in descent from Fiacha Suidhe, a brother of Conn Ceadchathach (of the Hundred Battles). an early Deisi Mumhan genealogy:   (Rawlinson) Gilla Pátric m. Máel Sechnaill m. Gillai Brigte m. Domnaill m. Fáeláin m. Cormaic m. Mothla m. Ruadrach m. Donnchada m. Dúnchada m. Éogain m. Bregduilb m. Cumascaich m. Cobthaich m. Áeda m. Fintain m. Cláire m. Cainnich m. Ernbraind m. Niad m. Briúin m. Éogain Bricc m. Airtt Chirp m. Cairpri Rigronn m. Fiachach Suigde m. Feideilmid Rechtada, son of Tuathal Teachtmhar.