111Grierson, Helen. "Tara." The Irish Monthly, Vol. 29, No. 336 (Jun., 1901), pp. 326-330.
In another article in the same journal, Alice Curtayne quotes from O'Connell's speech that day, in which he indicated that he "...had chosen Tara for the meeting because it was the spot where the chieftains of Ireland bound themselves 'by the sacred pledge of honour and the tie of religion to stand by their native land.'" (Curtayne, Alice. "The Twilight of Tara." The Irish Monthly, vol. 60, no. 706, 1932, pp. 190-91.)
Other sources estimate the crowd at Tara's Monster Meeting to be up to 750,000 or even 1 million people. More than 20 years afterwards a ballad was composed to commemorate the event. (https://digital.nls.uk/broadsides/view/?id=20795)