Amelia,
I think what you have found was repeated across Ireland (and Europe generally). The first folk to leave Ireland wrote home once they were settled in the US, describing the conditions they had found, the price of everyday items, and what the local job prospects were. Consequently folk at home were well informed about what they might find if they decided to emigrate. And they preferred to head for somewhere they knew someone rather than striking out blindly. So that's why you get people from the same area living near each other in the US.
PRONI (the public record office in Belfast) has a vast collection of such emigrant letters. An example being D627/213B dated 14th Jan 1866 from Alex Aitken of Rapid Creek, near Iowa City. He wrote to William Patterson in Fivemiletown, Co Tyrone describing his experiences. A summary being:
Letter. Did not stop in Canada as land too dear at £8-15£ per acre. Has brought 80 acres at 9 per acre. Would return if Mr Montgomery offered "a" situation worth coming for". Would rather be in Ireland but U.S.A. is a money making place. Details of crops, etc. Endorsed by Patterson with a description of Aitken.
