Every UCAS or Irish university application is to one specific named course -- Economics, Chemical Engineering, whatever it is -- and that's what you're evaluated on, admitted into, and (usually) study for the whole degree. There's no "undeclared" box to check, no first-year lineup of intro classes across departments before picking a lane. The choice you make on the application is close to the choice you're committing to.
Changing your mind after you've started is a real process, not paperwork with your advisor
UCAS's own guidance is direct about this: transferring to a different course "varies by university and course," typically involves filling in a declaration or a full application form, and can include an interview or a request for additional work to prove you're ready for the new subject. Course credit doesn't automatically carry over either -- if the new course is similar enough, some of what you've completed may transfer, but if it's a genuinely different subject, you may need to restart from the beginning of first year rather than just picking up where you left off. Compare that to a typical US school, where changing your declared major is often a same-semester conversation with your advisor -- there's no equivalent to that low-friction option built into most UK or Ireland degree structures.
Joint honours: the closest thing to a US double major
If you want to study two subjects together, the mechanism for that is a joint honours or combined honours course -- a specific, separately coded application (e.g. "History and Politics" as its own course, not "History" plus a minor), not something you add on after enrolling in a single-subject degree. Some universities do allow a switch from joint honours into a single-honours version of one of the two subjects at the end of first year, provided you meet that subject's own progression requirements -- but this varies significantly by university and isn't something to count on going in. If you know you want to study two subjects together, apply to the joint course directly; don't plan to bolt one on later.
Different courses at the same university can have very different requirements
This app's own calculator data already shows this in practice: a Medicine or Economics course at a given university routinely asks for a meaningfully higher SAT/ACT figure, or a more selective grade profile, than a Humanities course at that same institution. That's not a data inconsistency -- it reflects real, course-specific demand and selectivity that a single university-wide "how hard is it to get into X" number would flatten out. Check the chances calculator for your specific target course at a school, not just the school's general reputation.
What this means for you
- Treat your course choice as close to final before you apply -- switching later is a real process with paperwork, possible interviews, and a real risk of losing progress, not a routine advising conversation.
- If you want to study two subjects, apply to a joint/combined honours course directly -- don't plan on adding a second subject after you've already enrolled in a single-honours degree.
- Research your target course's own entry requirements, not just the university's -- Medicine and a Humanities subject at the same school can sit at completely different selectivity levels.
- If you're genuinely unsure what you want to study, take that seriously as a reason to think harder before applying -- see Should You Apply Abroad? The Real Tradeoffs for how much this shapes whether studying abroad fits you at all.
For how this plays out in each system specifically, see How UCAS Actually Works and How CAO Actually Works.