United flight operated by Aer lingus

Partner airlines will have different flight numbers for the same flight. You can tell if it's the same flight if the departure and arrival times are identical.
If you will give the cities and dates involved you can get a better response here regarding the prices and possible alternatives.

If the plane is leaving at exactly the same time and date, and if the terminology is "United flight xxx operated by Aer Lingus", then it is the same plane. It is a code share flight. A code share flight is one which is operated by one airline, but marketed by others. Under such circumstances, each airline generates its own flight code. Thus a single flight might be identified by more than one flight code. For example, the same flight might be both United flight xxx and Aer Lingus flight xxx. The flight code on your particular ticket will depend on which airline you booked your ticket through.

Marge, nice to see you're doing your homework and doing some research before posting your question and looking for that lower fare! In 2010 I bought a ticket through United, the flight was from Washington DC to Madrid, but I actually traveled on Aer Lingus, what the previous two posts stated is exactly right. What was interesting was the aircraft was Aer Lingus, the pilot was Irish, but the flight attendants were Americans, they also wore Aer Lingus uniforms. When I asked why it was this way she told me the United wanted the route but didn't have the proper aircraft for it so they used the Aer Lingus aircraft.

It will be the same flight. I assume you'll be making a plane change somewhere in the US. The first leg of your trip will be on United, then when you get on the plane that actually takes you to Ireland, you'll be on an Aer Lingus plane. The flights of the same time/date listed on the Aer Lingus site are the same as those listed on the United site so go with the less expensive option. When you check in with United, you will receive all the boarding passes for the outbound flights. By the way, the service on Aer Lingus when I traveled was excellent. The meal is huge too and rather tasty for airplane food.

Thank you so much for your help. I have booked the flights with the lowest fare. Now on to the planning!