I think, but not yet fully checked that Ireland may not be where this (expected) Margaret was born, maybe 2 different people?
take alleged son James 1807-1877, on his death in 1877, it says he is married to Helen/Ellen Dempsey, you can follow this James (born in Ireland through the Censuses).. Ellen was his 2nd wife, so perhaps Agnes McCabe/McCaig was his first?
on his death there is no parents names given, on his marriage to Ellen/Helen Dempsey his parents are James Frew (a Farmer, not a Coal master) and an Agnes.. (can't read the last name but it's not Margaret Tennant)
I can find no connection with a Samuel born 1804 in Ireland, so if you take Ireland out of the equation (for now).. tbc..