As the situation is ever-changing, it can be hard to understand how it could impact your holiday.
The reason for the 'self-isolation' requirement from the Irish Government is, of course, to safeguard public health.
Therefore it wouldn't be an additional public health benefit if you travel via Belfast, as you would still be coming from off the Island of Ireland.
Based on this, I wouldn't recommend putting yourself to the additional effort of taking the longer round-trip.
In essence though, the expectation is that you would self-isolate by restricting your movements for 14 days upon your arrival. So perhaps the best approach is to look at what you can do at your parent's place, or in the immediate vicinity that keeps contact with other people to a minimum.
Furthermore, know that in the interests of their staff and other customers, some tourism businesses may ask that you don't use their services if you've arrived within the previous 14 days.
Another recommendation I would have for you is to download the Irish COVID-tracker app for the period of your stay. It'll help with notifying people if you or anyone you have proximity to subsequently get diagnosed.
As you are planning your trip for next month, please make sure to keep an eye on the latest information regarding the regulations here
All of this is not ideal aand not what we'd wish for your holiday here.
Let us know if you would like some help on finding things to do during your stay with reduced contact with others, though I am sure your family may also have plenty of recommendations for you.