There are plenty of Donohues around here, 10 alone in my job's address list, though I don't know any of them in particular.
If you are interested, there are many sites for searching for ancestors, it seems to be a quite good business for Irish/Americans. However, there really is nothing like feeling yourself what that ancestry means, and the best place for that is to roam around some villages and heritage centres of Ireland that capture the spirit of the emmigrants, specially at the time of the famine and the coffin ships. But prepare yourself for a blow, if you open your mind and heart to the memories that surround you it is an emotionally demanding experience.
A funny place in my opinion is the Great Blaskett Island and its heritage center (off the Dingle peninsula, in Kerry). No-one lives in the island any more, the last residents were evacuated sometime in the 50's, I think. The island was however a vibrant community of over 1,000 people in the late 1800's. It strived because of the famine - in the island at least there was fish. The remains of the village and the heritage centre preserve the memory of what it was like to live there, and the faces and stories of the last residents. You sort of go there, and you no longer need to wonder why people emigrated from Ireland any more. Specially if you go on a rainy day (which has a 90% chance of occuring, whatever the time of the year...)
For stronger guts, there's a heritage centre in Skibereen, West Cork, which preserves the memories of the famine, in one of the areas of Ireland that was worst hit.
Convince all your friends to come to Ireland too. The best is in the west - West Cork, Kerry and Galway. Tourism is the biggest industry of Ireland and America a main market, which dropped 30% after Sept. 11. Disclaimer: the previous sentence is an advertisement on behalf of the Kerry Tourism Board.