Well, here at the bottom are lexicons-think of them as roots and soil.
"The Mettle of the Pasture"
James Lane Allen
The word Sithbhe is indeed given in both lexicons, but explained a city, not a round tower.
"Thomas Davis, Selections from his Prose and Poetry"
Thomas Davis Commentator: T. W. Rolleston
They weighed two hundred and twenty pounds apiece and were eligible to the All-American team, but you couldn't make the human lexicons look at it that way.
"At Good Old Siwash"
George Fitch