I am sure I am way late on this one, but basic policy for unchangeable/change fee tickets (ie, cheapest you can find) is that you can use the unused return for up to one year on that airline only. You can change the ticket, but it will cost you upwards of at least $100 per change and then you will also have to pay the difference.
Example, if you buy a return for $400 to N Ireland with Aer Lingus you'll have $200 worth of return to use on any Aer Lingus flight for one year (they do include the taxesk, which is nice). Of course, your change fee and difference in cost will be added on.
Great thing about this is that you can use the unused return for a flight to somewhere else in Ireland or Europe and have a weekend holiday. Bad thing about this is if your job is over one year, you have just thrown away money. So buy a single ticket.