Question: Cynfarch Oer, was probably a 6th-century king of the Sub-Roman realm of Rheged, was one of the kingdoms of the "Hen Ogledd" ("Old North"), the Brittonic-speaking region of what is now Northern England, and southern Scotland, during the post-Roman era, and which time period?

Context articles: Meirchion Gul
Cynfarch Oer
Kingdom of Strathclyde
Coel Hen
Thirteen Treasures of the Island of Britain
Urien
Llywarch Hen
Hen Ogledd
Rheged
Bonedd Gwŷr y Gogledd

Meirchion Gul was probably a late 5th-century king of Rheged, a Brythonic realm in the area of sub-Roman known as the "Hen Ogledd" (in the North of England and Southern Scotland today).
Cynfarch Oer ('Cynfarch the Dismal', also known as Cunomarcus or Cynfarch ap Meirchion) was probably a 6th-century king of the Sub-Roman realm of Rheged, believed to be located in north-west England and south-west Scotland.
Strathclyde (lit. "
Strath of the River Clyde"), originally Cumbric: Ystrad Clud or Alclud (and Strath-Clota in Anglo-Saxon), was one of the early medieval kingdoms of the Britons in "Hen Ogledd" ("the Old North"), the Brythonic-speaking parts of what is now southern Scotland and northern England.
 The kingdom developed during the post-Roman period.
 It is also known as Alt Clut, a Brittonic term for Dumbarton Castle, the medieval capital of the region.
 It may have had its origins with the Brythonic Damnonii people of Ptolemy's "Geography".
Coel (Old Welsh: "Coil") or Coel Hen ("Coel the Old") is a figure prominent in Welsh literature and legend since the Middle Ages.
 Early Welsh tradition knew of a "Coel Hen" (Coel the Old), a "circa" Fourth Century CE leader in Roman or Sub-Roman Britain and the progenitor of several kingly lines in the Hen Ogledd ("the Old North"), the Brittonic-speaking part of northern England and southern Scotland.
 Later medieval legend told of a Coel, apparently derived from Coel Hen, who was the father of Saint Helena and the grandfather of Roman Emperor Constantine the Great.
 Other similarly named characters may be confused or conflated with the Welsh Coel.
 The legendary "King Coel" is sometimes supposed to be the historical basis for the popular nursery rhyme "Old King Cole", but this is unlikely.
The Thirteen Treasures of the Island of Britain (Welsh: "Tri Thlws ar Ddeg Ynys Prydain") are a series of items in late medieval Welsh tradition.
 Lists of the items appear in texts dating to the 15th and 16th centuries.
 Most of the items are placed in the "Hen Ogledd" or "Old North", the Brittonic-speaking parts of what is now southern Scotland and Northern England; some early manuscripts refer to the whole list specifically as treasures "that were in the North".
 The number of treasures is always given as thirteen, but some later versions list different items, replacing or combining entries to maintain the number.
 Later versions also supplement the plain list with explanatory comments about each treasure.
Urien , often referred to as Urien Rheged or Uriens, was a late 6th-century king of Rheged, an early British kingdom of the "Hen Ogledd" (northern England and southern Scotland).
 His power and his victories, including the battles of Gwen Ystrad and Alt Clut Ford, are celebrated in the praise poems to him by Taliesin, preserved in the "Book of Taliesin".
 He became the "King Urien of Gorre" of later Arthurian legend and his son Owain mab Urien was later known as Ywain.
Llywarch Hen, meaning 'Llywarch the Old' (born c. 534, died c. 608), was a prince and poet of the Brythonic kingdom of Rheged, a ruling family in the Hen Ogledd or "Old North" of Britain (modern southern Scotland and northern England).
 Along with Taliesin, Aneirin, and Myrddin, he is held to be one of the four great bards of early Welsh poetry.
 Whether he actually wrote the poems attributed to him is unknown, and most of what is known about his life is derived from early medieval poems which may or may not be historically accurate.
Yr Hen Ogledd (] ), in English the Old North, is the region of Northern England and the southern Scottish Lowlands inhabited by the Celtic Britons of sub-Roman Britain and the Early Middle Ages.
 Its denizens spoke a variety of the Brittonic language known as Cumbric.
 The Hen Ogledd was distinct from the parts of northern Britain inhabited by the Picts, Anglo-Saxons, and Scoti as well as from Wales, although the people of the Hen Ogledd were the same Brittonic stock as the Picts, Welsh and Cornish, and the region loomed large in Welsh literature and tradition for centuries after its kingdoms had disappeared.
Rheged (] ) was one of the kingdoms of the "Hen Ogledd" ("Old North"), the Brittonic-speaking region of what is now Northern England and southern Scotland, during the post-Roman era and Early Middle Ages.
 It is recorded in several poetic and bardic sources, although its borders are not described in any of them.
 Some modern scholars have suggested that it included what is now Cumbria in North West England and possibly extended into Lancashire and Scotland.
 In some sources, Rheged is intimately associated with the king Urien Rheged and his family.
 Its inhabitants spoke Cumbric, a Brittonic dialect closely related to Old Welsh.
Bonedd Gwŷr y Gogledd (English: The Descent of the Men of the North ) is a brief Middle Welsh tract which claims to give the pedigrees of twenty 6th century rulers of the Hen Ogledd, the Brittonic-speaking parts of southern Scotland and northern England.
 It is attested in a number of manuscripts, the earliest being NLW, Peniarth MS 45, which has been dated to the late 13th century.
 The text may have been composed in the 12th century.
 The historicity of much of the information is spurious or in doubt.
 Although certain parts are in agreement with the earlier Harleian genealogies, the text represents a substantial revision seeking to integrate the branches of many rulers and heroes who are prominent in other traditions, such as the Rheged prince Llywarch Hen.