Some achievements are dead-easy, while others are going to take some work.
Enter the fabled fish of Ironforge, stage left. [Old Ironjaw] is spoken of only in whispers, deep in the dwarven taverns. Rumors speculate that he is uncatchable, but this will not deter you. After all, you rival the legendary Nat Pagle, Angler Extreme with your fishing prowess.
Getting down to business, it is important to set out prepared. If you are a member of the Alliance, all you will need to do is cast your rod into either the waters of the Mystic Ward or the Forlorn Cavern in Ironforge, and expect to be there for awhile. Don't worry about your actual fishing level; skill is for chumps.
For the Horde, the achievement Old Ironjaw will be much more difficult. Preparation will require the affectation of humility. You must accept that you will die and that you will probably have to grovel. Try donning your best fishing or fancy holiday garb to appear as harmless as possible.