I don't know of Captain Dennis Donahue's role in the Fenian movement. But, from research i have found that Lieut.Dennis Donahue, commanded a cavalry patrol(12th Tn Cav.USA) that captured and later shot my 3rd G-Grandfather in Dickson County,tenn. as a guerrilla.

I searched Google Books; and found a reprinted newspaper that was published:

Mentions Colonel Dennis Donahue; in command the 5th Maryland Cavalry Reg't (part of the Fenian army of invasion)
http://books.google.com/books?id=CSkpAAAAYAAJ&pg=PA95&dq=%22Dennis+donahue%22%22Fenian%22&hl=en&ei=yMEMTqqYPIP20gGr79CDDg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCoQ6AEwAA#v=onepage&q=%22Dennis%20donahue%22%22Fenian%22&f=false

Best Regards,
John