I just looked at the UCD horizons website and its seems like it's the same as Trinity's Broad Curriculum -ie- gives students a chance in a particular faculty to study subjects outside their own area of study.
All the degrees at Trinity and UCD are based on credit systems (ECTS - European Credit Transfer System) it's just you don't get any choice in the credits you study. Horizons seems to give you some control over some of the credits that you study each year.
For medicine the whole system isn't really amenable because it is a professional course that has a fixed number of subjects required to be studied in ordered to be competent.
I really doubt UCD would move to a GPA system... A few years ago there was talk that the NUI (ie- UCC, UCD, UCG, Maynooth) may change their grading system because of the low number of First Class Honours degrees being awarded by the NUI. Again, this isn't really applicable to medicine as a pass is 50% as opposed to 40% in most other faculties. Moving to a GPA system in medicine would just confuse the whole situation...