Dál Riata or Dál Riada (also Dalriada) (/dælˈriːədə/) was a Gaelic kingdom that encompassed the western seaboard of Scotland and north-eastern Ireland, on each side of the North Channel. At its height in the 6th and 7th centuries, it covered what is now Argyll ("Coast of the Gaels") in Scotland and part of County Antrim in Northern Ireland. After a period of expansion, Dál Riata eventually became associated with the Gaelic Kingdom of Alba.
In Argyll, it consisted of four main kindreds or tribes, each with their own chief: Cenél nGabráin (based in Kintyre), Cenél nÓengusa (based on Islay), Cenél Loairn (who gave their name to the district of Lorn) and Cenél Comgaill (who gave their name to Cowal). The hillfort of Dunadd is believed to have been its capital. Other royal forts included Dunollie, Dunaverty and Dunseverick. Within Dál Riata was the important monastery of Iona, which played a key role in the spread of Celtic Christianity throughout northern Britain, and in the development of insular art. Iona was a centre of learning and produced many important manuscripts. Dál Riata had a strong seafaring culture and a large naval fleet.
Dál Riata is said to have been founded by the legendary king Fergus Mór (Fergus the Great) in the 5th century. The kingdom reached its height under Áedán mac Gabráin (r. 574–608). During his reign Dál Riata's power and influence grew; it carried out naval expeditions to Orkney and the Isle of Man, and assaults on the Brittonic kingdom of Strathclyde and Anglian kingdom of Bernicia. However, King Æthelfrith of Bernicia checked its growth at the Battle of Degsastan in 603. Serious defeats in Ireland and Scotland during the reign of Domnall Brecc (died 642) ended Dál Riata's "golden age", and the kingdom became a client of Northumbria for a time. In the 730s the Pictish king Óengus I led campaigns against Dál Riata and brought it under Pictish overlordship by 741. There is disagreement over the fate of the kingdom from the late 8th century onwards. Some scholars have seen no revival of Dál Riatan power after the long period of foreign domination (c. 637 to c. 750–760), while others have seen a revival under Áed Find (736–778). Some even claim that the Dál Riata usurped the kingship of Fortriu. From 795 onward there were sporadic Viking raids in Dál Riata. In the following century, there may have been a merger of the Dál Riatan and Pictish crowns. Some sources say Cináed mac Ailpín (Kenneth MacAlpin) was king of Dál Riata before becoming king of the Picts in 843, following a disastrous defeat of the Picts by Vikings. The kingdom's independence ended sometime after, as it merged with Pictland to form the Kingdom of Alba..