Hmm. The "But" here makes it sound like this knight didn't find what he was looking for. We can't be sure yet, though.
We do know that he "grew old," which indicates that he spent most of his life searching. It's almost as if he's wasted his life and is now an unsatisfied old man.
The order of the phrases in these two lines is slightly confusing, though. "This knight so bold" sounds kind of like an afterthought. We get the "he" in line 7, but not specifically who this "he" is until line 8. It's like the knight is no longer the super-fly hero that he was back in the first stanza.
And o'er his heart a shadow Fell as he found No spot of ground That looked like Eldorado.
The bad times roll on! It seems that, just like our man Bono, this knight still couldn't find what he was looking for.
What's more, he seems really bummed about that, hence the shadow "o'er his heart." (Fun game idea: next time your favorite sports team loses, you can say, "O'er my heart a shadow has fallen." Your pals will love it!)
The syntax of this line is a bit funky. In normal conversation, we would say a "shadow fell over his heart," but in poetry sometimes the order can be reversed, as it is here: "o'er his heart a shadow / Fell." ("O'er" is just a shortened form of "over." For example: "Mom, I'm going o'er to my friend's house.")
"Looked like" is interesting choice of words to us, too. Was the knight looking for Eldorado itself, or just any old "spot" that resembled it? It's hard to say.
Perhaps the ambiguity implies that the knight was looking for something that was the equivalent of Eldorado, even if not exactly the Lost City of Gold. Maybe that something could even be abstract, like emotional fulfillment.