The Youghal where your Darcys are from is indeed located in Tipperary, located just northwest of Nenagh, on the shores of Lough Derg. You can access a map of Tipperary and Youghal at the Lonely Planet website link at:http://www.lonelyplanet.com/maps/europe/ireland/
You'll have to enlarge the map of Tipperary several click before Youghal comes into view.
There is also a Youghal in County Cork. I believe Sir Walter Raleigh was once the Mayor of Youghal, Cork.
Kate, I really do hope you can make it to Nenagh and Youghal someday. I've spent many happy days in Nenagh, as my cousins are from there, and have lived there since my cousin Joe Harrington became a Garda in the 1940s.
Again, please write with any new or exciting information that you may find.