Photo credit: National Trust for Scotland, Hermiston Quay
There is quite a similarity with William Dobson's portrait of Montrose after which there is a line engraving in the Scottish National Portrait Gallery: https://www.nationalgalleries.org/art-and-artists/35684/james-graham-1st-marquess-montrose-1612-1650-royalist
However, that print is probably eighteenth century, not seventeenth century.
George Vertue's engraving of the same portrait (an impression of which is in the British Museum) was made in 1731: http://www.britishmuseum.org/research/collection_online/collection_object_details.aspx?objectId=3606136&partId=1&searchText=montrose&page=1
The National Trust for Scotland would be interesting to know more about it. The previous director of the Scottish National Portrait Gallery, James Holloway, had commented a few years ago that he believed the painting could be by Dobson but was, at the very least, from the circle of Dobson.
your portrait appears to be a copy of the original Dobson portrait sold at Sotheby's in 2013:
Much of the debate regarding the various portraits of James Graham, 1st Marquis of Montrose, can be found here:
As mentioned therein, a useful discussion on the multiplicity of very different images of James Graham - from four paintings that, it is claimed, bear the greatest resemblance to him, and on to "the endless engravings of the penny-print school" - can be found in the first appendix of the first volume of the 1856 edition of "Memoirs of the Marquis of Montrose" by Mark Napier. The attached composite shows these four "true likenesses" as described in the book.
Attached also are two other composites depicting Graham.
I agree with Rab MacGibbon: this portrait is definitely of copy quality. While apparently deriving from the work sold at Sotheby's in 2013, it lacks the latter's confident handling of the face, and the painting of the collar and the armour is more laboured.
This is clearly after the Dobson portrait sold by Sotheby's linked above, so the original question would seem to have been answered. Also, the Art UK entry should give the sitter's vital dates (1612-1650).