Hi Bill,
The Dutch connection is a red herring. If you've ever actually looked at the Moylagh/Oldcastle parish registers, you'll see that the dounble 'a' spelling is fairly rare and rather accidental, and the most common spelling is Grall. And for most of the 19th, the same registers record this name as Fitzgerald. So it looks like Grall/Grall is a corruption of the Irish form of Fitzgerald, which is MacGerailt. Sounds like you don't want to believe this, but the evidence speaks for itself!