Maura (This is my daughter's name - it's Irish for Mary, so kind of Irish and kind of Biblical at the same time.)
Maren (I think this one sounds nice with both Josiah and Sean. It's not Irish, so Josiah isn't left out as the only non-Irish name, but it's not quite Biblical, either (though it could also be a nod to Mary). It has Mar- like you wanted.)
Since Hubs loves Isabella, have you considered Isadora or Arabella? Or Annabelle, which is quite Southern.
Some common Irish girl names without convoluted spellings:
Bridget (from the goddess Brighid)
Liadan (nn Lia?)
Sinead - I know we're treading into Gaelic-y waters here, but you could call her Jane!
Maureen (from Mairin)
Suanach - perhaps an alternative to Susannah? It is quite Gaelic, even though it's pronounced as spelled.
Aisling - perhaps my favorite "common" Irish name, as it means "dream or vision"
One suggestion I have is a variant of Rose: There's the Gaelic Roisin (ro-sheen) and slightly anglicized Rosaleen, as well as Rosalind, Rosalie, Rosamond, and the spunky nickname Ros.
Names that hopefully fit one or more of your personal criteria:
Josephine (for all the reasons you love Josiah, plus Josephine March and the spunkiness of Jo on a girl)
Cornelia or Cordelia
Charlotte (for the South, and vintage nn Lottie)
Magdalen (clunky, quirky, and an Ma- name)
Abigail (Biblical and very similar in feel to Agatha)
Philomena OR Wilhelmina (as in Mina Harker!)
Victoria (classical, but underused)
Theodora or Dorothea
Augusta (perfectly Southern name with a lot of history)
Elmira or Almira
Hadassah (vintage Biblical name that gives me a similar vibe as Susannah)
Stella (or Stelmaria if you're feeling totally adventurous!)
Zipporah or variant Sephora (One of my favorite Bible names...too out there?)
Hope this helps :)