Cillian can remember a great deal of numbers because he groups them all in sets of 3 and thinks of them as batting averages. He is using the technique of .

Explanation:

This technique basically involves the division of the whole data or information under consideration into small fragments or bits and then attributing each event or memory to these set of information that we want to memorize in the mean time we will get an idea of the whole process and its importance during our professional life as well.