It's funny... I've been having a similar conversation elsewhere about baby names. I'm a big fan of unique names, especially ones that have a long history. But I dislike "popular" names, or ones that are "over the top" in ethnicity. Most people tend to think of their child's name in the context of schoolyard bullying, but it's also important to think of the name that he/she will carry as an adult. Imagine them being a doctor, lawyer, politician, etc. Would they be taken seriously with the name, or would it be a hindrance to them?

That said, I like Declan. I even like the Gaelic spelling of it. Having to correct people is not that big of a deal if you only use it as a "legal" spelling. For common usage, the child could use an Anglicized spelling. It's no different than other nickname variants like John for Jonathan, or Mike for Michael, or Bill for William. Lots of people sign their names for unofficial/non-legal stuff using nicknames or alternate spellings. So that would always be an option, even if the legal name is Gaelic.