I have no reason to believe that the "Picts" were other than P-Celtic-speaking "Brythons" who were cut off and isolated by the Romans (through the Hadrian and Antonine Walls and forts/campaigns between and beyond them). The Brythonic "Picts" would have occupied all of Scotland (north of the Solway-Firth line), including the Western Isles, before the migration of the Irish Gaels ("Scotti") to Argyle (Dalriada) c.500 CE.
The ancient I2a-L126 sample (c.1311 BCE) on an island, Pabay Mor, off the west coast of the Isle of Lewis, Outer Hebrides, at least raises the question of a post-Bell Beaker I2a "refugium" (I2a-M284>FGC14213>L1195>L137>L126) in the Western Isles, the descendants of which assimilated to the incoming R1b Celts expanding from the Halstadt/La Tenne Culture in Central Europe (c.750 BCE?).
Thus, my quizzical reference to the possibility of "West Picts" (Epidii?). We know little of them because
1) the Romans had little interaction with them, unless they were the fabled "Attacotti" of the "Barbarian Conspiracy" (c.367/68 CE), which threatened to push the Romans out of Britain and put the Romans permanently on the defense, eventually leading to their withdrawal from Britainia (c.410 CE) as not worth the expense to defend.
2) they would have later been assimilated by the Irish Gaels/Dalriada (c.500 CE).
We do know more, through the Romans of the Celtic tribes to the east (Caledonii/Maetae, or "East Picts"?) and the Brythonic tribes to the south (Damnonii, Votadini, Selgovae, Novantae).
The most recent common ancestor of L126 > FGC20063 > FT2393 > FT2393 > S7753 > Y4142 >Y4751 is estimated by FTDNA's Discover tool to have been born with a higher likelihood ~353 CE, corresponding to a decade or so before the Barbarian Conspiracy (367-68 CE), with sons (aged in their 30s?) around the time of Rome's withdrawal from Britainia (c.410 CE).
Y4751, with 21 branches in the I-M223 Project hosted by FTDNA, is indicative of a sudden "star burst" expansion following a long bottleneck:
L126 (3) > FGC20063 (2) > FT2393 (2) > FT2393 (2) > S7753 (2) > Y4142 (3) > Y4751 (21)
The "star burst" looks to spread from the Isles/Highlands (my Big Y matches at Y4142 is primarily from the Western Isles/Highlands) to the Lowlands, Borders, Yorkshire, Wales, and Ulster.
This looks more like a overlayer (superstrate) rather than an underlayer (substrate) poking through in spots.
Many Isles/Highland "clan" surnames and Lowland/Borders "family" surnames have minority I2a lineages among the majority R1b lineages. Scottish clans/families and tribes are not homogeneous, but heterogeneous. In western Scotland, the Y4142>Y4751 expansion could have come from a founder's effect that burst forth once the lid was taken off the pot, so to speak. In ("east") Pictland proper, while I2a might form a thin overlayer, an R1b "Bell Beaker" underlayer (substrate) could subsist under a main R1b "Celtic" (Halstadt/La Tenne) body.
The northern "Celtic" tribes would have encompassed the Isles (Epidae?), Highlands (Caledonii/Maetae ), Lowlands/Borders (Damnonii, Votadini, Selgovae, Novantae), Midlands (Brigantes), etc., and likely would have come into the "Pritanic Isles" during the Iron Age (c.750 BCE) as an expansion of the Halstadt/La Tenne Culture, which developed from the Urnfield Culture. They would have encountered peoples who were already there - Bell Beakers (R1b), but also possibly remnants of a Pre-Bell Beaker Neolithic/Mesolithic population (I2a). The Bell Beakers (bronze-using) and Celts (iron-using) invaders had technological advantages over the resident "native" populations. The pre-Bell Beaker population had been largely "replaced" (but not eliminated), while the pre-Celtic population would have "assimilated" to new overlords and language. Over time Q-Celtic (in Ireland) and P-Celtic (in Britain) diverged due to separation - a theory is that Q-Celtic is the more "archaic" due to isolation, while P-Celtic was more exposed to influences from the continent.
