Scheduled air flights from Europe:
Air Europa, Air France, Condor/Thomas Cook, East Line Airlines, Iberia, LTU, Lauda Air in addition to Martinair Holland.
All these airlines frequently offer discounted prices, especially in off peak season travel times.
This is for all the airports in the Dominican Republic. You will need to check which ones fly out of POP. Check Condor and LTU for starters.