Thanks,
@Package4 , you summed up my thoughts, exactly. I posted the original image over on the Civil War Naval Forum FB group, and there was strong agreement that this is a Marine Corps uniform of the 1859 pattern. One person there wrote, This is the 1859 Officer Dress Uniform for a Lieutenant. The shoulder scales and thickness of the wool tassels is also correct for the period and rank. Reference is "The Civil War Uniforms of United States Marine Corps, The regulations of 1859 LtCol Cureton and David Sullivan."
I'm also thinking that the name "Cunningham" may be incorrect, which is what's causing the confusion.