Starting with the 5th edition of General Stud Book Vol. 1, 1891, (p. 13), the WHITESHIRT MARE was represented as a half-sister to MOTHER WESTERN, (the grandam of Eclipse), their dam by Old Montagu - mare by Hautboy - mare by Brimmer - Royal Mare [GSB 1:13].   Mitochondrial DNA typing confirms that modern tail female descendants of the Whiteshirt Mare (MRCA - Sister 1723c to Sampson by Greyhound) are not matrilineally related to Mother Western's descendants but the connection was questioned long before it was possible to test it scientifically.

By dates alone, it's just about impossible that the Whiteshirt Mare and Mother Western were half-sisters. Since Whiteshirt Mare was grandam to Flanderkin, foaled in 1706, she was 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 Flanderkin were covered as two year olds, would be 1700. Also assuming that the Montague mare was covered at two and her first foal was the Whiteshirt mare, then Montague Mare would have been 30 when Mother Western was born. Addressing this matter in Family Tables of Racehorses Bobinski & Zamoyski assign the date of 1731 to Mother Western, giving a difference of at least 34 years between her and the Whiteshirt Mare.

As entered GSB 1, ed. 4, 1858, in damside pedigrees of tail female descendants, the Whiteshirt Mare's dam was identified only as "Montagu Mare", no pedigree given, no association with the family of Mother Western suggested. This is consistent with earlier records from other sources but lacking a detail noted therein that is critical to disambiguating the Montagu[e] Mares in the Whiteshirt Mare and Mother Western pedigrees.

C M Prior, citing 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]" [Early Records:37].

William Pick says WHITESHIRT MARE was out of "a favourite mare of Lord Montague's of Cawdry, Sussex" [Pick 1:22].

Citing Prior and Pick, Bobinski & Zamoyski (op.cit.) also suggest the imminently plausible scenario that "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". 

Bobinski & Zamoyski further offer that the Whiteshirt Mare might have been of the same matrilineage  as Sedbury (1734 Croft's Partner) of family 68, the taproot of which was his second dam, a mare of Lord Montague's.  While possible and plausible, there is nothing in the historic record, other than association with Lord Montague, to make that connection; neither can it be tested by mtDNA typing since family 68 is long extinct in tail female.