Green hills, long daylight hours, and coastal drives make Ireland worth the trip. Heading over between May and September gives you the best shot at mild weather, open country roads, and lively evenings in the towns.
A typical round-trip flight from the US costs about $600. If you do not want to take that hit to your savings all at once, you can spread the cost out evenly without paying extra in finance charges.
Remember that a 0% intro APR is a repayment deadline, not free money. If you still have a balance when the 15 months end, regular interest applies to whatever remains, so set up automatic monthly payments to clear the balance on schedule.