The Celtic tribal groups had exchanged their cultures and language across the Irish Sea from the earliest days. The Irish, taking advantage of Roman withdrawal, competed with the Scots, Picts, and Saxons, for a piece of Welsh territory. Inscribed stones document this exchange of culture, which became scattered about the landscape of Wales. [Ogam inscriptions, some with Latin and Ogam.] It was the threat of Irish invasion that brought Cunedda from the Men of the North to help save Gwynedd! [The northern most territory in Wales.]
Things had pretty much settled down by the Norman invasion. The Celtic Church had adopted the Roman Church way of life [hair cuts, calender, marriage, and the like] except for a few of those fringe elements that seemed to want to remain a little more independent. Hadrian IV, the first and only English Pope (1154-1159 AD), decided to give Ireland to Henry II. This Pope essentially wanted Henry II to conquer Ireland and bring this island under the influence and guidance of the Church in Rome. Henry II directed Richard de Clare, Earl of Pembroke, nicknamed Strongbow, to take a leadership roll in this endeavor. Strongbow took a large number of Welsh archers and Welsh cavalry to Ireland. These folks were essentially private adventurers from Wales seeking lands of their own. This invasion was most successful around the city of Dublin, and English law was established as in an English shire. The area around Dublin became know as "the Pale". Under Edward I, Dublin, Waterford, and Cork became the major trading towns under English (was actually Norman) control.
It was during this time time that many of the Welsh families settled in Ireland. Here they began their families' tradition among the Irish. Of course the Welsh kept some of their culture, language, and naming system which was always a puzzle to the Normans, Anglo-Saxons, and ultimately what became the English. Thus the Welsh brought to Ireland their frequently used Christian name JOHN. The Church, using Latin, would spell it IOAN. The Normans using French would spell it JEVAN. The Welsh using their language would spell it SION. The Anglo-Saxons using their language spelled it JOHN. Those Welsh who mostly came to fight were under Norman influence and adopted JEVAN. The Irish would spell it "SEAN" (Shawn) The literature of the time records both "ap Jevan", "ap John", and in many cases "ap Ievan". Thus begins the JONES surname in Ireland. [More to come!]
For reference to Ogam see:
"A History of Wales", by John Davies, p. 47.
"The Celtic Realms", by Myles Dillon and Nora Chadwick, pp. 38-40.