We had decided on two names before we got our St. One was Earnest, and the other Seamus. We thought Earnest was so befitting of the gentle but intimidating St. Bernard, but I loved the name Seamus. We decided to let the dog decide. There were two pups we loved. A definite Earnest who was cuddily and sweet, and then our little man Seamus who was independent and aloof. We decided on Seamus b/c he was the largest and had by far the most beautiful markings. So, I guess my answer is technically we decided before we got him AND the minute we saw him.
I do love his name so much, and i'm happy we picked it. The one thing I don't like though, we picked the traditional Irish spelling as opposed to the phonetic spelling Shamus. It really confuses people, and they always call him SEE-MUS. Completely understandable b/c that is how it looks, but it still bothers me!