As others have said, the irritation will lessen as you get used to it. Remember how a real cigarette made you choke when you first smoked that? You got used to it, and you will get used to this.
That said, some e-liquids are harsher than others. the Loong Totem/E-Cig.com formula, which most manufacturers use for their cartridges, is the harshest. Try ordering carts or liquid from Janty, e-cig-sales or ePuffer. It's less harsh. Or, for the smoothest vapor, try Freesmoke. Expensive, but VERY smooth.