TASK DEFINITION: You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For consistency, answers should be in all lower cased letters.
PROBLEM: Category: ENDS IN "FF" 
Clue: The heaviest of dog breeds, it can exceed 180 pounds

SOLUTION: mastiff

PROBLEM: Category: POTPOURRI 
Clue: Country in which you'd find the Book of Kells

SOLUTION: ireland

PROBLEM: Category: FRIENDLY PHRASES 
Clue: He is "a friend indeed"

SOLUTION:
a friend in need