Evan Blayney [Ieuan Blaeney] (1370-1430) of Tregynon (Powys). "Although his ancestors had lived in the area between Newtown and Welshpool for several centuries, being off-shoots of the princely Powys line, he was the first to have used Blaeney as a surname. His name appears on the Burgess Roll for Welshpool in 1406" 
Maelienydd/Kinsham Blayneys: Robert Blayney states... "a family living in Radnorshire, in the mid 1400s at Maelienydd adopted [the Blayney] name. Similar in status to the other Blayneys but totally unconnected. They later were at Kinsham, and in the late 1800s the principle representatives of this family lived in a house called The Lodge on the edge of the town of Evesham. As far as I know, these Evesham Blayneys had no male near relations."  S.P. Thomas confirms this view and also states this Blayney family "became extinct in the male line in the last century [ie 1800s], apart from any descendants of younger sons from the Kinsham period or earlier" . Essentially, this meant "That in the abscence of any clear indication to the contrary, one is justified in assuming that any male Blayney (however spelt) deriving from Cedewain or its environs [during the 17th and early 18th centuries], might well be a descendant from Evan of Tregynon." 
However, in 1834 John Burke Esq. believed that Meilir Gryg (about 19th in descent from Brochwel Ysgithrog), was ancestor of all the Blayneys in Britain and Ireland. . There is some evidence that both the Gregynog and the Radnorshire Blayneys can trace back to Meilir Gryg but DNA evidence doesn't support a strong relationship.
Hywel/Ieuan Blaene b~1370. Peter C Bantrum's "Welsh Genealogies, AD 300-1400"  has a Y Dean DU of Llananno (b~1500) ab Owain ap Meredudd ab Ieuan FYCHAN ab Ieuan Blaene. This line did not continue the Blayney surname beyond two generations. 
While the Baronal Blayneys of Monaghan were landed and strongly Protestant, Blayney and Blaney families from other parts of Ireland were Catholic and in the main poor with no known geneaological link to the Blayneys of Montgomeryshire. ,  There are many 'Blanes' in Scotland who may have a common source as there has been migration between Ireland and Scotland over the centuries. The Celtic Blaenau in its various forms meaning "top" or "source" would have been Anglicised to Blayney. This tendency can be seen in the use of "Blayney" (on maps & papers) to describe the historic Carmarthenshire home "BLAENAU, Llandybie" .