Although you have the option of playing on either Classic or Impossible difficulty it will be much easier on Classic. Ironman mode means you have only one save file that autosaves after both you and the aliens have had your turn. Although the enemies will be challenging, the main difficulty you will face with this mode is the possibility of making a mistake or wrong decision and being decimated within a few turns, unable to reload a save allowing you to correct this.
You will need to have a solid understanding on how the game plays and what strategies work in each situation. It is highly recommended you attempt a play through on Classic difficulty prior to starting Ironman Mode, as trying to go from Easy or Normal to Classic/Ironman Mode can be overwhelming.
See 'Our Finest Hour' for more on how the difficulties differ.
For a detailed walkthrough/strategy guide on beating Classic/Ironman Mode refer to the Classic/Ironman Mode Walkthrough
You can also find additional gameplay and Ironman mode tips HERE