There is an interactive map of Ireland at http://interactive2.iol.ie/MAPS/content/iolmap.asp
When you enter "Thomastown" in the Town or Region field and select County Tipperary, it will bring up 10 possible locations. All but one are in the area between Tipperary Town and Cashel.
BTW I have just started a search for John Farrell and family who emigrated from Thurles, Tipperary, to Chicago area about 1873. Any connection to your O'Farrell???