According to the General Stud Book, volume one, 5th edition, page 13, this Whiteshirt mare was half-sister to Mother Western, the grandam of Eclipse, and was out of the mare by Old Montagu, her dam by Hautboy - mare by Brimmer - Royal Mare. Family #12.
Early Records of the Thoroughbred Horse, by C M Prior, page 37, in the private stud book of Mr Cuthbert Routh, says the Whiteshirt mare was - 'out of an extraordinary mare of Ld Montague of Codrey [Cowdray, Sussex]'. William Pick's Turf Register, volume one, page 22, says she was out of - 'a favourite mare of Lord Montague's of Cawdry, Sussex'. Since she was great great great grandam of Hanniball, who was foaled in 1713, she is most unlikely to have been half-sister to Mother Western who had her last recorded foal in 1757. The earliest date of foaling for Mother Western, assuming she was 30 when she had her last known foal, would be 1727; and the latest date of foaling for the Whiteshirt mare, assuming she and each mare between her and Hanniball were covered as two year olds, would be 1698. If we also assume the Montagu mare was covered at two and her first foal was the Whiteshirt mare, then she would have been 32 when Mother Western was born. The Family Tables of Racehorses, by K Bobinski, says Mother Western was foaled in 1731, which makes the difference 36 years.
K Bobinski, in a note to family #12, says - 'The compiler of the 5th edition of the GSB, Vol. 1, probably confused a Mare by D'Arcy's Old Montagu with a mare of Lord Montague; whose name was also spelt Montagu. There can be no doubt that the WHITESHIRT MARE and Mother Western descended from distinctly separate roots, the former tracing to the breed of Lord Montague of Cowdray, and the latter to one of the D'Arcy Royal Mares'.