We have the same story from my Granny. Mary Bridget O'Brien (n. Seery) b. 1905 at Grange Hall, Ballynacargy, Co. Westmeath , to Sylvester Seery and Margaret Moran. Known siblings Jack (?John) and Maggie (Margaret).
Would love to hear what information you have to refute this tale of royal decent. Where did it start? Perhaps, Alexs' cousin John is right. I know do know that Granny and Rose Kennedy (of THE Kennedy's) were first cousins.