As long as the bottles have remained sealed they should be safe for at least another 6 months. Bailey's used to advertise a 30 month shelf life, with the same 2 year quality guarantee they still give. The alcohol content will act as a preservative against most microbial spoilage, but there is still a possibility of rancidity from other sources. If the product has been exposed to direct sunlight, heat above 25˚C, or temperatures below 5˚C you may see a separation of the whiskey-cream emulsion. Even under the best conditions the emulsion will break eventually yielding a separation between the whiskey and the fats from the cream.
The best thing to do is to keep it in a cool, dark place and check the product for any issues of odor, appearance, and flavor as each new bottle is opened.