Hi redeyednewt,
L21 is a big group. I think it is now safe to say that it represents Insular Celts, i.e., the people who appear to have repopulated Great Britain and Ireland in the Bronze Age. There's evidence that the Neolithic farmers (Y-DNA haplogroups mainly G2a, H2, I2a1, and I2a2a) were almost completely replaced by R1b-L21 men. They likely originated in southern Germany, and are found in low frequency throughout western continental Europe. It's likely that a kindred of L21 migrated en masse to the European Atlantic Zone, including Great Britain, Ireland and the Atlantic coasts of France, Spain and Portugal. Through famine, war, and/or pestilence, they may have found Great Britain and Ireland largely depopulated. Filling the void, they exerted a strong founder effect on the genetics of Great Britain and Ireland seen to this day. These L21 men were likely associated with the Bell Beaker culture, who were metal workers and ore prospectors that traveled widely across western Europe, including the Atlantic Zone. They likely brought Indo-European languages with them.
In my own Scottish L21 case, through next generation sequencing and SNP (single nucleotide polymorphism) discovery, I found a cousin whose lineage is from the interior of northern Portugal. Our common ancestor likely lived some time between 1,700-450 BCE (there is a lot of variance among the several dating methods now being used), I tend to lean on the archeological evidence that shows vital cultural links between Iberia and the British Isles existing earlier rather than later in that time frame. The Romans encountered and conquered the Celtic-speaking Galleci/Calleci people of northern Spain and Portugal, that became the Roman province of Gallaecia, that became modern-day Galicia and Portugal.
The L21 lineages that repopulated Great Britain and Ireland proliferated into a large number of subclades. These were the P-Celtic-speaking Britons, Picts, and Q-Celtic-speaking Scotti, whom the Romans encountered, colonized, and/or walled off at the the beginning of the Common Era.
They were also the inhabitants of Great Britain that the Anglo-Saxons invaders fought and displaced along the southeast and east coast (The Jutes settled in Kent and Hampshire, the Saxons in the rest of southern England as far north as the Worcestershire, while the Angles occupied the rest of England from East Anglia to Northumberland and southeastern Scotland). Some Britons displaced westward. Some Britons survived the conquest, remained in-placed and were assimilated into the new Anglo-Saxon dominant culture and forgot their Celtic speech. Some Britons fled the Anglo-Saxons across the English Channel, particularly to Brittany, with a small number also fleeing to Galicia.
Then aside from the Anglo-Saxon newcomers, the Britons, Picts, Scots, Welsh and Irish, were the inhabitants of Great Britain and Ireland, in the middle to late medieval periods, that the Vikings encountered, fought, and in several places (the Shetlands, Orkneys, Yorkshire, Northumbria, Dublin, Wexford, coastal northern & northwest Scotland and the Islands) dominated, before the Viking elites were ultimately assimilated. Same for the conquering Normans and their mercenary elite from Brittany (a reintroduction of some L21 blood lines) and surrounding areas.
So, today L21 lineages are largely associated with much of the Scottish Highlands and the Islands, southwest Scotland, Northern Ireland, Ireland, northwest England, Wales, Cornwall, and to a somewhat less frequent extent the rest of England, as well as the Diaspora of these people in United States, Canada, Australia, and New Zealand.
To find out more precisely about where you and your lineage fit into this saga, you would need to do more refined testing.
I hope this helps.