My own family comes from the Ballymahon (Longford/Westmeath) area which is the only town which fits the descrioption in your post. You may be slightly exaggerating the number of Bannons in the area but not much. My name is Seamus Bannon and I am related to a William and Thomas Bannon who migrated to Australia in the 1840s or 50s. If they are the same ones I would be interested in any information you might have and will help anyway I can.
Many thanks
Seamus (seamus.bannon@gmail.com)