Lennon, J. (1988). Fairs and Assemblies in Ireland. "Before I Forget…": Journal of the Poyntzpass and District Local History Society, 2, 55–62. http://www.jstor.org/stable/25512034

Basically his argument is:

• The Tailteann Games predate the Olympics by five centuries.

• A manuscript mentions that there were Greek traders at the Tailteann Games.

• Therefore it’s possible the merchants travelling back to Greece brought the idea back to them and that became the Greek Olympics

Reasonable enough chain of thought, but totally speculative. I rate it 10% probability.