Re The Countess of Beauregard, formerly Harriet Howard, born Eliza(beth) Ann Haryett of Brighton (1823 -1865)
Unless Major Mountjoy Martyn's wife and mistress were clones of each other, the picture you have is almost certainly his red haired mistress Eliza(beth) Haryett formerly of Brighton, who the major's circle of wealthy Londoners and titled aristocrats knowingly addressed as his wife.
Although your picture has a date of 1839, the probable age of the lady, the quality of her clothing and her acquired jewelery makes me think it may be from a little later. In 1839, Eliza was only in her late teens and better known as the actress Harriet Howard, and at the most, had only recently come 'under the major's protection' and been accommodated at a separate address from him in Mayfair.
Re: Two other known images of Mrs Mountjoy Martyn.
I leave it to you to decide whether it is possible that your picture is is predated by one used in the Countess of Blessington's book on Health & Beauty published 1841. I believe the picture of Mrs Mountjoy Martyn shown on the following website comes from that:
http://www.old-print.com/cgi-bin/category.cgi?item=B3761870214 It shows Eliza as a voluptuous female with poise, beauty and adorned by simple trinkets, perhaps explaining why so many wealthy admirers were attracted to her.
The picture you have is better suited to being a few years on, for a more mature Eliza now sports the expensive (but in my opinion a little tasteless) jewelery that had been showered upon her, possibly as rewards for her favours after her association with the major had waned. These may have been the very jewels she later sold in order to fund Napoleon III's return to France.
Lastly there is the magnificent portrait of her as the Countess of Beauregard (Wikipedia
http://en.wikipedia.org/wiki/Harriet_Howard ) by which time this simple shoemaker's daughter from a Brighton backstreet had risen from poverty, and having negotiated repayment for her earlier loan (and perhaps discreteness?) gained a title and a French Chateau. In that final picture she somehow seems to radiate the satisfied grace and opulence of those deemed to have come from an aristocratic bloodline.
I have seen it suggested that due to Napoleon IIIs rekindled interest, her unfortunate early demise was engineered by a representative of the French government
"Napoleon III and the women he loved", but no evidence to substantiate that has ever emerged. Roy G