I don't know any Declans in real life, and DH and I just had a baby boy we named Declan. After our baby's name was announced at my job, a coworker emailed me to tell me he and his wife planned to name their baby boy Declan early next year. He was checking to make sure that was okay with me (which he totally didn't need to do but was a very polite gesture). That's the only other Declan I've heard of--and they may end up changing their baby's name before he's born.
I have an aversion to extremely popular names, and I had the same fear. DH and I talked about it at length before we decided to roll with it anyway. My heritage is Irish, and DH and I both love Ireland. I wanted to honor that, and the name had a lot of meaning for us both. I don't think it's going to be the next Aiden, although I could be wrong. I really hope it remains out of the top 100.