As the previous poster just noted, tell the truth. Just tell them you're doing it for the miles. Period. End of story.
Without meaning to stereotype, the Irish are, IMHO, incredibly hosptiable. The last time I did an MR to Ireland, the gate agents were simply sad that I wasn't going to get to see their beautiful country. They so wanted us (my fellow MR'ers and I) to experience Ireland, and were sad for us that we weren't. I assured them I had been before, loved it, and would go again, but that this time, it was just for the miles.