What Lynn said. (Sound Bay Newfs)
A Landseer Newfoundland is a WHITE dog with black markings in various patterns.
An Irish Spotted Newfoundland is a BLACK dog with white markings in specific areas...the toes, tip of tail, chest (any or all of those areas).
There is a specific gene for Landseer. There is no specific gene for Irish Spotted. They're genetically black with white markings. Interestingly the white markings show up in the same places all the time (toes/foot, chest, tail tip)...so for some reason those genes are expressed in a specific way. The white on the chest is usually a fairly large (or very large) area that's obvious.
You can have a pure black dog who is Landseer recessive who may have a spot or so of white (generally on the toes, or chest), but the markings aren't as uniform as Irish Spotted.
Janice's Georg and Magnum are what are considered Irish Spotteds. They're black with the white markings as indicated.
There are some "dark" Landseers, where the black markings cover the majority of the torso ... but genetically, they're Landseers. The white undercarriage and generally more white on the legs, etc, is often a hint, but sometimes, it's hard to ascertain without a genetic test.
And occasionally you'll see a pure black dog with a small white patch on it's chest when young. When they're grown, that patch is gone. Frankie (left...avatar) had a small white patch, and now, it's three hairs on his chest and I can't even find them unless I'm working through the coat combing to the skin. Same goes for our Addie. I know that he has no Landseer in his pedigree. Not sure about Irish Spotted. I haven't had his DNA tested for color so far, but I do know his grandmother is grey so he may well be grey recessive.
And as far as what can be shown in the US, it's right in the standard.
Color
Color is secondary to type, structure, and soundness. Recognized Newfoundland colors are black, brown, gray, and white and black.
Solid Colors--Blacks, Browns, and Grays may appear as solid colors or solid colors with white at any, some, or all, of the following locations: chin, chest, toes, and tip of tail. Any amount of white found at these locations is typical and is not penalized. Also typical are a tinge of bronze on a black or gray coat and lighter furnishings on a brown or gray coat.
Landseer--White base coat with black markings. Typically, the head is solid black, or black with white on the muzzle, with or without a blaze. There is a separate black saddle and black on the rump extending onto a white tail.
Markings, on either Solid Colors or Landseers, might deviate considerably from those described and should be penalized only to the extent of the deviation. Clear white or white with minimal ticking is preferred.
Beauty of markings should be considered only when comparing dogs of otherwise comparable quality and never at the expense of type, structure and soundness.
Disqualifications-- Any colors or combinations of colors not specifically described are disqualified.
In Canada, Brown and Grey cannot be shown.