I really don't understand the Aer Lingus site! I could almost book the Houston to Dublin portion (330 days in advance), but I could not book Houston to BCN or the JFK to BCN on one ticket.
You CAN book IAH to DUB or JFK to DUB now. (JFK to DUB was about $407 one way)
You CAN book the DUB to BCN with the same 330 days in advance for about66 euros one way. They have the two flights a day to BCN. One 6 am-ish and 6 pm-ish.
But when I tried to get prices for the entire JFK-BCN run, I got this weird note:
Flights do not operate on your selected travel date/s. Aer Lingus operates from Apr 14 2007 - Mar 27 2010 on days Mon Tue Wed Thu Fri Sat Sun
I wasn't sure if they were weren't going to offer the flight to BCN after 3/27 or what. I was-- and still am--very confused.

Maybe the layover of many hours was too long to be considered one ticketed flight?
I liked Air Lingus as you could book a reasonably priced one way ticket.
I am afraid of buying a round trip and throwing away half as I know it is technically against the rules of carriage for some airlines and with my luck I would be the one that they would choose to make an example of!!!
I've looked at Vayama and Cheapoair and they have some really good deals. I just wish I knew how reliable they are.