I go back and forth on this one. Unlike the names Tiana, Aurora, and Arielle, I have never heard the name on real person. But unlike Cinderella, Rapunzel, Mulan, etc...Merida seems much tamer and more usable, and as far as I know doesn't have any major folk-lore or historical connections. I would say that in a few years, it will be usable - but people will probably associate it with the princess and make comments here and there, but I wouldn't expect a lot of teasing or eye rolls from it.
I don't think the tie is that strong actually. I have actually asked people this, and they didn't remember the characters name was Merida. It not the biggest Disney movie, and I think the ties are very weak. Especially in 5-10 years.
This in my top name lists, and I would definitely consider it usable.