TLDR– Ystrad Clud was a Welsh/ Briton kingdom, they spoke Cymbric. They had to fight against the Scots, the (English) Anglos, Irish raiders and then the Vikings. At one point, the kingdom was conquered by the English king and given to the Scots as a reward, although it was later taken back by the Cymbric king. Viking attacks weakened Ystrad Clud's defences, and they were eventually conquered by the Scots and the land became part of Scotland. Gaelic replaced Cymbric language and culture and they came under Scottish rule. That area is now called Strathclyde, which is the Gaelic version of the name.
In the past, Britain was inhabited by the 'ancient Britons/Welsh' who were tribal, but spoke a common Brythonic language and shared culture.
By the 9th century, a lot of the land had been taken by the Anglo Saxons. But there were lands/ kingdoms that were still ruled by the Britons/ Welsh. There was Wales/ Cymru and Ystrad Clud/ Strathclyde and Cornwall/ Kernyw.
You can see from this map that Welsh owned land (Ystrad Clud/ Strathclyde) stretched up into where we now call, Scotland. The people of Ystrad Clud spoke Cymbric, which was similar to Welsh/ Cymraeg.
https://www.medievalists.net/2012/11/monasteries-and-the-geography-of-power-in-the-age-of-bede/
As you can see from this map, Dal Riata was a Gaelic kingdom that encompassed parts of northern (where we now call) Scotland and Northern Ireland.
Dal Riata, conquered the Picts and merged to become 'Alba'.
During this time, Ystrad Clud was dealing with attacks and raids from the Irish, the Scots, the Vikings and the Northumbrian Anglos.
And by this time, parts of Yr Hen Ogledd (The Old North) became a part of Northumberland under Anglo (English ) rule.
Excerpt from an article about the Lost kingdom of the Britons
It may come as a surprise that another kingdom existed far longer than Scotland, spoke in a tongue alien to modern, even Gaelic ears, and was located across what is now western, southern Scotland, the borders with England, and much of Northern England. At one point the boundaries of this great, now lost Kingdom stretched from the northern shores of Loch Lomond to the Yorkshire dales. Today Scots have forgotten that this lost Kingdom of the Britons, a post-Roman state, predating Gaelic, tartan, bagpipes, Bannockburn, the stone of destiny, and all the other medieval and Victorian tropes of Scottishness.
The Venerable Bede, an English Benedictine monk in Northumbria, wrote in his Ecclesiastical History of the English People (in AD 731) that “there is a city of Britons, highly fortified to the present day called Alt Cluid, which in their language means the Rock of the Clyde”.
It is sad that the Britons from the Rock of Dumbarton, who were power brokers, fought in numerous tribal battles, played host to religious men of the North, such as Kentigern or Saint Mungo, are long forgotten, hidden in the mists of dark historical time.
The Britons found themselves surrounded by a merged Gaelic speaking Scottish tribe and the Picts on one side, emerging as Alba (Scotland) represented by St Andrews Saltire’s flag. On the other side was the ever-powerful presence of Christianised Northumbria in the Lothians. The Britons lands were ever-shrinking. They had to battle Picts, do deals with the Scots and Northumbrians, or won temporary respite when their enemies fought each other. The beginning of the end came when Norwegian kings (aka Vikings) besieged the Brythonic capital, for four months, until their well dried up and those in the fortress capital were starving. The Annals of Ulster record how “a great host were taken into captivity”. King Athral of the Britons was killed and as the Vikings retreated the Scots began their take over, and Gaelic started to be spoken in the Kingdom. The new overlords, the Scots, gave the name Strathclyde to the Britons.
The Britons of the Rock disappear from history in the 11th century, and Scottish history takes over.
And
The kingdom of the Strathclyde Britons ended in c.1070 when it was conquered by the Scottish king Malcolm son of Duncan (Mael Coluim son of Donnchad). Its royal dynasty was deposed, never to be reinstated, and the native aristocracy had to submit to Malcolm or flee into exile. Those who remained had little choice except to embrace the Gaelic language and culture of their conquerors to eventually become ‘Scots’ themselves.
https://senchus.wordpress.com/2010/05/29/clan-galbraith/
At one point, Ystrad Clud was conquered by the English and was given to the King of Scotland as a reward.
Following the battle of Brunanburh, Owain's son Dyfnwal ab Owain became king of Strathclyde. It is likely that whereas Scotland allied with England, Strathclyde held to its alliance with the Vikings. In 945, Æthelstan's half-brother Edmund, who had succeeded to the English throne in 939, ravaged Strathclyde. According to the thirteenth-century chronicler Roger of Wendover, Edmund had two sons of Dyfnwal blinded, perhaps to deprive their father of throneworthy heirs. Edmund then gave the kingdom to King Malcolm I of Scotland in return for a pledge to defend it on land and on sea, but Dyfnwal soon recovered his kingdom.
The end of Ystrad Clud, and how Cumbria became part of England
King Arthgal ap Dyfnwal, called "king of the Britons of Strathclyde", was killed in Dublin in 872 at the instigation of Causantín mac Cináeda.
Some time after 1018 and before 1054, the kingdom of Strathclyde appears to have been conquered by the Scots, most probably during the reign of Máel Coluim mac Cináeda who died in 1034.[17] In 1054, the English king Edward the Confessor dispatched Earl Siward of Northumbria against the Scots, ruled by Mac Bethad mac Findláich (Macbeth), along with an otherwise unknown "Malcolm son of the king of the Cumbrians", in Strathclyde. The name Malcolm or Máel Coluim again caused confusion, some historians later supposing that this was the later king of Scots Máel Coluim mac Donnchada (Máel Coluim Cenn Mór). It is not known if Malcolm/Máel Coluim ever became "king of the Cumbrians", or, if so, for how long.[18]
The Keswick area was conquered by the Anglo-Saxon Kingdom of Northumbria in the seventh century, but Northumbria was destroyed by the Vikings in the late ninth. In the early tenth century it became part of Strathclyde; it remained part of Strathclyde until about 1050, when Siward, Earl of Northumbria, conquered that part of Cumbria.[19]
Carlisle was part of Scotland by 1066, and thus was not recorded in the 1086 Domesday Book. This changed in 1092, when William the Conqueror's son William Rufus invaded the region and incorporated Cumberland into England. The construction of Carlisle Castle began in 1093 on the site of the Roman fort, south of the River Eden. The castle was rebuilt in stone in 1112, with a keep and the city walls.
By the 1070s, if not earlier in the reign of Máel Coluim mac Donnchada, it appears that the Scots again controlled Strathclyde.
As in England where Saxon/ English culture replaced Welsh/ Briton culture, Gaelic replaced Welsh.
So this Welsh/ Briton land became part of Scotland. The same way Welsh/ Briton land became part of England.
Scots/ Gaelic laws replaced Welsh laws. Early Scots laws show that the Britons were a separate group within Scotland.
https://en.wikipedia.org/wiki/Leges_inter_Brettos_et_Scottos
In the Declaration of Arbroath (a letter written in 1320 to the Pope, asking him to recognise Scotland's independence) boasts of the Scots 'driving out the Britons' i.e the Welsh.
Occidente quas nunc optinet, expulsis primo Britonibus
The Britons it first drove out
by leopardprintcactus