Robert was the son of Richard Guinness (1755-1829), a Dublin barrister and judge, and his wife Mary Darley, descended from a well-known Dublin house-building family.
He married firstly Mary Anne Seymour in November 1822, who died in 1837 at their home in Stillorgan, County Dublin.
His granddaughter Lucy Guinness was the wife of the painter Philip de László.
Like his father, uncle and brother, Robert trained as a lawyer at the Kings Inns and was called to the bar in Dublin.
[2] In 1854 the firm moved to premises on College Green in the centre of Dublin, increasing its banking business, and adding lines in insurance and assurance.