Hi there
My Co Mayo ancestors were Burke/Flynn/ Derrig (Derrick) and they left for Scotland at the time of the Famine, before my branch went onto New Zealand.
This is a complete long shot- but I have a big group of DNA matches (18 of them at 4th-6th cousin level) topped with an O'Halloran 4th -6th cousin. It looks like some of his ancestors went to Chicago. I can't work out a common ancestor with this group, and have been thinking they might have a Co Limerick origin. However, as I do have some Co Mayo ancestors, I thought it was worth asking if your O'Halloran might possibly be linked with my family.
Margaret