What they said. :-) It's exceedingly difficult to draw anything other than very broad conclusions about the origins and migratory paths of haplogroups, either yDNA or mtDNA. They're simply too old. With yDNA NextGen testing like FTDNA's BigY, we're getting closer to identifying Y-SNPs that may be indicative of location when bifurcation occurs (bifurcation is when the mutation of a SNP first happens and a split from an older haplogroup occurs, forming a new one). But to bring these results into genealogical relevance--meaning to put them in the timeframe of adoption of surnames, in England circa 1100 AD--you have to dive down literally dozens of tested SNPs below M405 in the hierarchy.
And I may have given an incorrect impression with the first answer. I almost always (just like that) try to hedge my bets because the quasi-scientist in me knows that few things are absolute and most anything is possible. I said it was entirely possible your Pentland surname M405 could be traced back to Northern Ireland or Scotland because it is possible. That it was an early arrival in that region--early as in BCE years--is less likely.
M405--also know as U106 and S21: exact same SNP (or "reference cluster") just different names--bifurcated (you just knew I'd work that in again) from L151, which in turn, and in order, came from L51, L23, and M269. The yDNA phylogenetic tree of haplogroups is hierarchical, and it's also chronological. M405 is newer than L151, which is newer than L51, and so on. Today, M269 is estimated to be present in 80% or more of males in Ireland, and 73% of males in Scotland. M405, though, not so much. It's in about 6% of the Irish population and 12% of the Scottish. Here's a heat-map of M405/U106/S21 from Eupedia:
In the British Isles, you can see that its saturation is very much southeast to northwest. This is consistent with the arrival of the Saxons and the Angles in England following the departure of the Roman army. From the late 5th century through the Battle of Hastings in 1066, the Anglo-Saxons ruled over most of England, for a period grudgingly sharing it with the Norse during what's referred to as the Danelaw. Neither the Romans nor the Anglo-Saxons ever made much headway into what we now call Scotland, and virtually zippo into Ireland. The Irish Sea isn't very wide but, in part due to the "you ain't conquerin' me" spirit of the Welsh, neither of those occupiers ever gained a reliable enough foothold on England's west coast to consider invading Ireland.
Conversely, the recent DNA study of Irish populations I mentioned before showed the exact opposite saturation of Irish genomes. Every sample tested in Scotland showed elements of Irish background (the Irish supplanted the Picts in Scotland--in fact the early Roman term for the Irish was Scoti--and the Irish settled Scotland, not the other way around), and the Irish/Celtic DNA was just as clearly saturated west to east, from high concentrations in Ireland (no duh) to just a smattering toward England's eastern coast.
Historically, it seems Ireland became home to peoples who traveled there from the far western coasts of Europe and, later, by the Viking visitors who sailed around the north of Scotland. In fact, it was the Norse who founded the towns of Dublin, Limerick, Cork, Wexford, and others sometime in the early 900s. It was the Angles and the Saxons who carried the M405 SNP, not the peoples who occupied Ireland. If an Anglo-Saxon carrying M405 settled in Ireland, it likely was after 1175 and the Treaty of Windsor, and maybe even more likely after 1594 and the Nine Years War. Bottom line is that your M405 line could be very old in Ireland, but it probably didn't arrive there early, and it has remained a small minority of the population. It's origins are more clearly Germanic and its highest saturations today are in the Netherlands and northwest Germany.
