You can book flights that don't originate in your home country. For example, I have my home country set to Ireland, but I frequently book flights that originate in Amsterdam or Heathrow. You need to select the 'All Countries' option at the top of the dropdown to lose the filter.
The reason you can't book DUB-ORK is because Aer Lingus don't fly it any more. Ryanair are the only airline on the route.