There are too many Reids everywhere. Belfast is partly in Antrim ad partly in Down, and people may have moved there in the later 18th or early 19th century for a while before deciding to emigrate further. Belfast was expanding at that time.
Best advice is to look at their neighbours in Iowa, and who they married. If all their neighbours andmarriage partners can be traced back to co. Antrim, then look more at co. Antrim, and see if you can plot on co. Antrim where those other people came from; Google their family trees, look at and post on other forums. Just googling Antrim and Mahaska turns up hundreds of connections.
I found this combination of james and Arthur Reid in the Belfast Newsletter index online, but it may be just a coincidence rather than an earlier generation of your family. However if some of these other people turn up in Mahaska Co. next door to Reids, then it might be a useful lead.BTW Nancy and Agnes are interchangeable versions of the name