In follow-up to a previous post, my Dad's Y-DNA test for SNP P78 came back negative, so Dad is not in the I2b1c subclade of I-M223.
So, with the other SNP's tested so far which have come back negative (M379, P95 and now P78), the only other I2b1 subclade he could possibly be in is the one found almost exclusively in the British Isles (as well as in Northeast Ireland where it is found among men with Gaelic surnames and in Western Scotland), namely I2b1a-M284.
Where M284 is found in Ireland, it is found particularly in baronial families with a credible pedigree back to a Cruithin (British) origin, suggesting an ancestor who arrived in Ireland from Celtic Britain:
For example it is found in McGuinness and McCartan men descended from the Uí Echach Cobha, a lineage considered Cruithin (refers also to the Scottish Picts) in the 6th century AD.
Dad's Y-DNA hasn't been tested for SNP M284 yet (I just ordered it). If Dad's Y-DNA tests positive for M284, it can also be tested for other more defining SNPs.
It's also possible that he doesn't have the "native British" M284 transition (the marker for subclade I2b1a) and is simply in haplogroup I2b1 (I-M223) without any of the defining markers of the currently known subclades of I2b1.
So, I will wait for the test results on the M284 SNP before making any conclusion, but I am leaning toward believing that if Dad's Y-DNA tests positive for M284, my Dad's fatherline may most likely be a (proto-Germanic influenced) pre-Celtic line (later becoming Celtic-Gaelic), whereas if his Y-DNA tests negative for M284, his paternal line may be more Anglo-Viking (originating in Northern Germany / Southern Sweden and coming into Britain during those invasions).
Whether M284+ or M284-, it is clear from the scientific research that my Dad's I-M223 Y-DNA fatherline is indigenous to Europe and arose from among the native European populations archeologist Marija Gimbutas calls "Old Europe."