Does any one know what ribbon I should use?

Find something available that isn’t on this list . You might need to do two colors, with a center strip or diagonal stripes, to differentiate yourself. If you establish it, make sure you update Wikipedia and anywhere else that you can show what it means.

Good luck on your fundraiser. I had never heard of Friedreich’s Ataxia before. And may I suggest always capitalizing it, you want to communicate that it is a serious matter.

It depends. Do you want it to represent Friedreich’s Ataxia and awareness to the disease or just your charity (your sstep father and the trip to Ireland)?
If it’s the latter, you could possibly find out where in Ireland he wants to visit and create a ribbon from the area tartan. If there is no particular destination or you’re unable to find out, I still like the idea of using an Irish plaid for the representation.

If you’re wanting to create and list an awareness ribbon for the disease… I’m not sure a plaid would be the best choice in that case. I do like the idea of an emerald green, though. To me, the solid color would be more universal yet the green would hold a special meaning for you.

I love the tartan plaid idea.