A very good question that—would that I had a better answer for it.

I have had little cause to study any theoretical ethnogenesis, unfortunately. I can tell you Traveller origins are not well understood, with many competing hypotheses; ranging from the pre-Celtic, to Cromwellian or Famine displacements. However, those that point towards origins in prehistorical or non-indigenous groups do not appear to be supported by genetic analysis:

Comparison with other European, Roma, and Indian populations shows that the Travellers are genetically distinct from the Roma and Indian populations and most genetically similar to Ireland, in agreement with earlier genetic analyses of the Travellers. However, the Travellers are still genetically distinct from other Irish populations, which could reflect some external gene flow and/or the action of genetic drift in a small group that was descended from a small number of founders.^1

Whatever its origins, Traveller culture is quite venerable. A ready anecdote to illustrate is that when the brilliant and prolific music collector Tom Munnelly chanced to meet a Traveller singer named John Reilly in 1965, Reilly would sing a song for Munnelly he called The Well Below the Valley. Munnelly, awe-struck, recognized the ballad as version of one that had been lost to the settled oral tradition for one hundred fifty years.

Traveller singing traditions were particularly vibrant, making them ideal sources for musically inclined collectors like Munnelly—though unfortunately those traditions have faded considerably today.

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^(1 • Relethford, J. H. and Crawford, M. H. Genetic drift and the population history of the Irish travellers. Am. J. Phys. Anthropol., 150: 184–189.)