My husband tested with familytreedna back in 2014 at the request of someone else with the same (not common) surname.
We had a name group forum going back then and this other person came back with an unusual result which didn’t match with anyone else in the group. His paper trail was almost identical to my husband originating in the same area of Limerick, same social standing etc., his line went to America/Canada and our line came to London.
Because of the unusual result indicating Native American heritage no less, our group name administrator suggested the least expensive test just to rule out the connection. I was positive it would be right unless there had been a NPE. The result came back with a match… dating back to 1720. As our line hadn’t been anywhere near America, we took some further SNP tests along with another match originating from Limerick who had done Y111.
The results came back showing that there was a genetic difference detween the native american and an European/Asian origin.
I should add the FTDNA administrator was also part of our surname group forum. Most people on the forum had done a YDNA test, the results were sorted into clusters by the administrator.
I would suggest you join FTDNA, and search for the name group for your grandfather first to see how it is organised. You can also email the administrator for that named group to ask for their advice, some are more helpful than others.
The tests are not cheap, and a general rule of thumb for a reasonably accurate match would be the Y111 test.
https://www.familytreedna.com/products/y-dnaWhen the results come back, check out your matches which you will see in your profile pages.