Marnane, Mernan, Mernin, etc ancestors

I am researching ancestors from Co Waterford who emigrated to Sangamon, Springfield, Illinois, USA in the 1850s, from Co Waterford. The name has been spelled variously  Mernane, Murnane, Mernon, Mernin, Mernan, Merning in the many and several records I have found, census and church records, English and Latin! Bridget and her brother Thomas married, had families and died in Sangamon. There's a headstone to Thomas who died in 1891, age given 72, and his wife Margaret - natives of Co Waterford. Bridget married Henry Farrant in 1860: they both died in the early 1880s, in Sangamon. I would be most grateful if anyone could shed light on the origins of this family, parish, townland, or any little tidbit at all. With very many thanks, Gay Conroy