they are concentrated in several areas, not all over; this is an extraordinarily rare name in the US. Besides, like so many other ethnic groups, some Irish choose names Americans recognize so their children can get along. I know tons of Mexican immigrants who speak NO English and name their kids
Justin,
Tiffany,
Brianna and
Brian for this reason, passing up
Luis and
Miguel. I believe few Americans would know how to pronounce
Niamh. Only a die-hard American nomophile, or someone who knows the Irish community (and that is NOT every American), would know the Gaelic pronunciation.