I most certainly would agree with you that Henry's work is indeed derivative.
The work of Paul Henry derives from and was influenced by the French movement, known as the Barbizon School, which moved art away from the depiction of decadent Parisian scenes and toward the depiction of rural scenes that were meant to capture the reality of the land. The artist who probably did this to best effect was Francois Millet. He painted the world of the French peasant, and the farm labourer.
Half a century later, Paul Henry used the depiction of peasants and rural labour to try to capture the reality of the land he lived in, Ireland.
His paintings of Connemara and Donegal, while repetitive, are consistent attempts to depict "Irishness" by using images of Irish rural labour, in the same way that the Barbizon school, and Millet in particular, used images of French rural labour to depict "Frenchness". Both types of images (the French and the Irish) have contributed greatly to the image of the respective countries conjured up in popular folklore.
Henry depicted "Irishness" using the same and similar subjects and technques to those used by Millet to depict "Frenchness", so Yes, I agree with you, Sandy, his work is most definitely derivative.
Beautiful, though, and costly.