Hi Kevin, I don't know the answer to your question on Flahive/Fahey surnames but you might try http://www.census.nationalarchives.iehttp://www.census.nationalarchives.ie for the 1911 Ireland Census as well as google the Irish Times surnames site to compare Flahive and Fahey surname variations and locations in Ireland.
Most of the Flahives I am following are in New York State (New York City + boros as well as Upstate NY) and came from County Kerry. I believe that in the Griffith's Valuation for 1852-54 there were 19 Flahive Heads of Households, 17 in Kerry, 1 in Limerick and 1 in Clare. The Griffith's valuation is free on the AskaboutIreland.ie web site where you will also finds tons of Fahey plus variations on that surname. Best of luck, Mary