He's in the Dictionary of National Biography, but with a record stating "architect, son of John Charles (1790-1851)", who seems to have an interesting life himself. Born in Dieppe of English parents and then imprisoned alongwith his parents during the war, he was educated in France and wrote French educational books. He was Master of French at Eton for many years, and I his sons were educated there too - one, Frank, became Master of French after his death.
Which is where the Rossmore link might come in - the Lord Rossmore who died went to Eton, the commissioning brother seems to have gone to Rugby instead... just need to check my dates. So it may have been personal friendship that got him the commissions. Henry Cairns Westenra, (b. 14 November 1851, d. 28 March 1874) 3rd/4th Baron Rossmore of Monaghan was educated at Eton between 1865 and 1868. Buildings were built in 1876.
Another small snippet : Edward John Tarver (1841-1891), architect, who also had aspirations as an illustrator and later became well known for his architectural renderings.
I'm really intriqued by this now - the Rossmores had used Morrisson and later Lynn to do the castle in the decades before - but why a relatively unknown English architect for these two structures?