I have heard that tansies will repel ants. Is this true and/or are there other plants that will work.
Tansy is not indigenous to North America, it is native to Europe and Asia.
As to it's properties for repelling insects, there is some mention of it being used to repel flies in the link article.
Here is a link that might be useful: Tanacetum vulgare
There are two native tansies -- T. camphoratum on the west coast and T. bipinnatum (Lake Huron/Port Huron tansy) on the shores of lakes Michigan, Superior and Huron and Maine rivers. I have no idea whether they'd have any insect-repelling properties.
T. vulgare expands by creeping roots and is not a good candidate for the garden. It's a fairly common roadside weed in northern Wisconsin, and I've seen it colonizing old fields.