A comment at Razib's blog cites data from 1972 about the historical dominance of the children of Irish clan leaders at the expense of commoners due to polygyny, a mechanism that could have been important in the rise of Bronze Age Y-DNA clade star-like expansions. It is a very logical way for this to happen, but I haven't previously seen solid anthropological evidence for this practice outside of Middle Eastern (especial Saudi Arabian) royalty.
Paul Ó DuḃṫaiġWith regards to ‘replacement’ and lineage driven socities, Kenneth Nicholls had the following to say in his 1972 book on ‘Gaelic and Gaelicised Ireland in the Middle Ages’https://www.amazon.com/Gaelic-Gaelicized-Ireland-Middle-Ages/dp/1843510030/ref=pd_sbs_14_1?_encoding=UTF8&pd_rd_i=1843510030&pd_rd_r=9405BAG59Y5XFE5B4VKE&pd_rd_w=XKMtC&pd_rd_wg=Qk03a&psc=1&refRID=9405BAG59Y5XFE5B4VKE($7.73 for kindle version of second edition)
“One of the most important phenomena in a clan-based society is that of expansion from the top downwards. The seventeenth-century Irish scholar and genealogist Dualtagh Mac Firbisigh remarked that ‘as the sons and families of the rulers multiplied, so their subjects and followers were squeezed out and withered away; and this phenomenon, the expansion of the ruling or dominant stocks at the expense of the remainder, is a normal feature in societies of this type. It has been observed of the modern Basotho of South Africa that ‘there is a constant displacement of commoners by royals [i.e. members of the royal clan] and of collateral royals by the direct descendants of the ruling prince;, and this could have been said without adaptation, of any important Gaelic or Gaelicized lordship of late medieval Ireland.
In Fermanagh, for example the kingship of the Maguires began only with the accession of Donn Mór in 1282 and the ramification of the family – with the exception of one or two small and territorially unimportant septs – began with the sons of the same man. the spread of his descendants can be seen by the genealogical tract called Geinelaighe Fhearmanach; by 1607 they must have been in the possession of at least three-quarters of the total soil of Fermanagh, having displaced or reduced the clans which had previously held it. The rate which an Irish clan could itself must not be underestimated. Turlough an fhíona O’Donnell, lord of Tirconnell (d. 1423) had eighteen sons (by ten different women) and fifty-nine grandsons in the male line. Mulmora O’Reilly, the lord of East Brefny, who died in 1566, had at least fifty-eight O’Reilly grandsons.
Philip Maguire, lord of Fermanagh (d. 1395) had twenty sons by eight mothers, and we know of at least fifty grandsons. Oliver Burke of Tirawley (two of whose became Lower Mac William although he himself had never held that position) left at least thirty-eight grandsons in the male line.
Irish law drew no distinction in matters of inheritance between the legitimate and the illegitimate and permitted the affiliation of children by their mother’s declaration (see Chapter 4), and the general sexual permissiveness of medieval Irish society must have allowed a rate of multiplication approaching that which is permitted by the polygyny practised in, for instance, the clan societies of southern Africa already cited.”
Given that such practises continued right up to 17th century in Ireland it’s not really surprising we see Star-Shaped Phylogenies in Irish kindred groups. It’s probably also part of reason why some lineages such as R1b-M222 underwent a starburst in western/northwestern Ireland in period after 400AD. (two of quoted individuals above are good candidates for been M222+The figures above omit daughters and granddaughters, so the total number of descendants of these clan leaders was almost twice as great.
Notably, the period in question precedes the arrival of the potato in Ireland and took place in a time of slow but steady population growth in Ireland following a major reduction in population in the 1200s CE probably due to plague and the Little Ice Age. So, it isn't that population was expanding dramatically. Instead, the clan leaders were squeezing out those less well off in social class who had fewer surviving children so that the leaders and their families could be fed.