A Stuart is auctioned, but is this William Brownlow the real Stuart?

I noticed a Gilbert Stuart portrait "The Right Honorable William Brownlow" was sold recently at auction at Sotheby's--my first question these days is "IS IT GENUINE?"  Now, I am not a titled expert.  But if one compares this portrait to the excellence of the portrait of Meeker, a question as to authenticity might be raised.  Does this portrait nail the likeness of Brownlow, or does it have an "artistic" look?  Could it in fact be a copy?
The estimate sale price was $15,000 to $25,000. and it sold for $27,500.00
Thus one should be certain that the artwork is genuine, and one would think that Sotheby's would know a copy from the real thing. OR. Might Sotheby's be interested more.... in a sale?  How much research does an auction house carry out on a particular artwork...

The portrait of Brownlow indeed is listed in the Lawrence Park volumes, with his image (however only in black and white.)  A swift comparison shows that the portraits are the same.

PROVENANCE (as provided by Sotheby's)
Sale: Heritage Auctions, Dallas, November 11, 2009, lot 66013
Acquired at the above sale by the present owner

The Right Honorable William Brownlow (1726-1794) 
by Gilbert Stuart c.1790 Dublin

FROM LAWRENCE PARK:
Dublin, c 1790.  Half-length, seated half-way to the right in an armchair upholstered in red.  His gray-blue eyes are directed to the spectator.  He wears a very dark blue velvet coat, a pale yellow figured or embroidered waistcoat, a white neckcloth and ruffled shirt.  His wig is powdered.  His right hand, holding a letter, is resting on a table covered with a soft gray-blue cloth.  The plain background is the color of dark oak.

A son of William Brownlow of County Armagh, Ireland, by his wife, Lady Elizabeth Hamilton, daughter of James, Sixth Earl of Abercorn.  In 1754 he married first, Judith Letitia Meredyth of Newtown, Meath; in 1765 he married, second, Catherine, daughter of Roger Hall of Mount Hall, Downshire, Ireland.  He was a Member of Parliament for County Armagh.  His grandson, Charles Brownlow (1795-1847) was, in 1839, created Baron Lurgan.

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The image in Park matches that which was auctioned, as far as I can tell.  But, if one compares the style of portrait between Brownlow and Meeker, Meeker appears to be so unbelievably accurate, to be almost a photograph. (My mother's dog Sammy used to look up at the portrait of Meeker and bark in the light of the setting sun!)  Possibly however Stuart DID have a different style in his earlier years in Dublin? But take a look at Aaron Burr, painted ca. 1794.

Aaron Burr by Gilbert Stuart ca. 1794

This is more of a Meeker style, not a Brownlow style.
And where is a more accurate provenance (history of ownership of the painting) of "Brownlow"?  Why is it lacking so substantially?  Where is the info that this portrait was, in the early 1900s, in the possession of Lord Lurgan of Brownlow House in Ireland (provided by Park)?

Reading Park more closely I found the following:


"A replica (or copy?) of this portrait is owned by Viscount de Vesci."