The interviewer is looking for a couple of main points in your answer. One, they would like to know that you have clearly researched the products / services offered at IRS. Two, they would like to see that you have actively thought about your potential fit with IRS and the ways that you would make a fast and positive impact on their organization.
Your answer should display your confidence and touch on your particular specialty. This could include your great ability to memorize product features, your natural sales abilities, or your cold calling skills.