Here is the Failte Ireland web site https://www.failteireland.ie/Commercial-development/trade-fam-sales-plat...
and in the middle of the page is a downloadable file with the names and contact info for approved tour guides. Some are local and others regional or national. National in this situation would be the Republic of Ireland. I did see at least one of the guides showing Northern Ireland also and likely there are more. I did not look at the entire list.
I doubt your "DNA cousin" would want to go to Northern Ireland but all you can do is ask.