Our 'Default Content Culture' setting is set to 'English - United Kingdom' with both 'Combine with default culture' and 'Combine files with default culture' options checked.
In SEO & URL's we don't currently have the 'Force domain culture' setting checked but the issue is resolved when we do check it. The problem then being, as you say, that we still require a combination of Translated pages with default culture. So when the .ie domain is hit. We want to serve the translated (Irish) pages where they exist and the UK (default) ones where they do not. Is this functionality what you are describing as needing custom code?
If so do you have any guidance as to what coding would be required?