Hi Audrey,
The first thing you'll probably want to do is check with rental companies for their existing policies and coverage. I assume that basic insurance is usually included when you rent a motorhome. Liability reduction options can often be purchased as an extra so that you can be sure to be comfortable with your liability and their cover limit.
There is a list of rental companies on
the Camping Ireland website.
If you feel like this isn't enough, your second best bet is probably to look into supplemental insurance back at home. Your existing car insurance company might be able to help you out, or your credit card company might offer insurance if you pay the rental company through them.
I hope this helps. Please do reply if you have any other questions. Another member of our community might be able to give some great advice as well!
Noor