there's multiple factors, some of which have to do with the US application process. Some have family here they'd like to be closer to. But I think what's increasing is the desire to see something new and experience a different culture (and in the case of Ireland, that means not having to learn another language as well). Just like you want to go somewhere else, the same happens with a lot of us. One of the admins I was speaking to think there's a correlation between this interest and the aftermath of 9/11. I'm not really sure if I believe that one, but there's definitely people who realize they want to see something new.
Also, I think there's an increase in popularity through word of mouth. A small majority of North Americans over here are Canadian - because of difficulties in getting into med school there, more Canadians have heard of studying in Ireland, and know of Irish grads who are successful in gaining a place in a Canadian hospital. Therefore, they equate Irish schools with success in learning medicine and being able to return home.
Each individual has their own reasons for wanting to come here. Just ask them 😉