I hadn't had (or seen) Ashmead's Kernel, Elstar, or Shamrock until I moved to Massachusetts. That might mean they need a cooler climate than you'll find in Maryland, or it may be random chance of the trees planted in the specific Hudson Valley orchards I got my heirloom apples from when I lived in New York. I hadn't had Elstar either, but learned from my post about them that they're named after a town in the Netherlands, so might be okay with a milder winter.

I wish I had a relative with an orchard, but my family are almost all city people (or suburban with back yards, and plants chosen mostly for appearance, ease of care, or to attract birds and butterflies).

...checking the ever-useful orangepippin.com... Ashmead's Kernel is an English apple grown widely in the U.S.; they say people are growing Elstar in Santa Cruz, CA, and Snohomish, WA; and they have little info on Shamrock. (All their "tree register" entries are anecdotal and thus incomplete.)