Many airlines allow you to book a single for slightly more (maybe €10) than half the cost of a return ticket. Certainly true of Aer Lingus and Alitalia. There's always Ryanair as well!
Generally cheaper to book directly with the airline than going through a consolidator.
Although Expedia is an excellent website, for some mad reason it will not accept Irish credit cards; I rang them up and they said it was to do with their bank and out of their control; sorry!!!