Several Robert Muirs are included on various Internet family trees; however, the Robert Muir, who was born in 1803 at Flemington, Lanarkshire, Scotland, is NOT the Robert Muir, who married Henrietta Brown on 26 January 1828 at Avondale, Lanarkshire, Scotland. The correct Robert Muir, was born about 1800 in Ireland, per the 1841 and 1861 Scotland Census. Both documents are available for purchase on ScotlandsPeople.
Henrietta Brown MAY have been born on 10 February 1812 to Peter Brown and Henrietta Scott. Her death date and location are unknown. She died before the death of her daughter Jean Muir on 19 August 1856 because Jean's mother was listed as deceased on the death registration. Henrietta's youngest son, Nathaniel Muir, was likely born in December 1850. So range of death for Henrietta (Brown) Muir is December 1850 to 19 August 1856.
Robert and Henrietta (Brown) Muir Family Tree (Ancestry.com -- paid subscription required; includes all information and copies of all records found to date)