This really is a question for your priest back home, as he is the one asking for information. As long as you are marrying in Ireland, local practices should apply.
As to registering as Catholic in Switzerland - is this something that you wish to do, or not?
If you wish to join the Swiss Church, simply go and speak to the priest at your local parish to get the ball rolling.
But if you do not wish to join the Swiss Church, think about why not, in light of your faith, and discuss this with your priest back home.
A different situation, but nonetheless one involving one of the sacrements:
When preparing to take on the role of godmother to a child back home, the priest in Chicago asked about how I practice my faith - as of course being a committed Catholic is a requirement, given that the role is one of religious guidance.
I simply explained that although in my heart I consider myself a Catholic in good standing, I nonetheless chose not to join the local Swiss parish because I believe that the bigotry and exclusivity that are hallmarks of this small village parish go against most of what the Church I was raised in stands for - and my conscience would not allow me to participate in this very anti-catholic (small c) Catholicism.
I explained how Swiss law requires one to financially support the local church if one declares membership, and that I, again in good conscience as a Catholic, cannot see my way to funding this parish's particular form of nastiness.
I then spoke of how I kept my faith alive without membership in the local parish. Cue a bit more philosophical/theological discussion - but my priest in Chicago was satisfied.
Now - my experiences as a member of a very liberal Chicago parish choosing not to join a Swiss small village parish are likely quite different from yours, both in Ireland and here. But take from that what you will.
On a practical note - if you choose not to join the Swiss church you of course cannot do the marriage prep course here. In that case, do the marriage prep course back in Ireland.
Thinking again about the marriage prep course - you might want to speak with the priest of the English Speaking Catholic Mission in Zürich: http://www.englishmission.ch
Now... I don't know the current priest or how inclusive or exclusive his views are. But many years ago the then priest was very open and inclusive. As this parish is sort-of-not-really part of the Swiss system, a group apart designed to meet the spiritual needs of English speaking foreigners - 15 years ago many of the congregation I knew were not officially registered with the Swiss Church - you might find guidance. Again, I do not know the current priest, or the current 'tone' of the congregation.
Wishing you all the best.