This portrait is of William Morgan the elder (1700-1731), the eldest son of John Morgan who was a Welsh politician of some note and of considerable influence. John also owned Tredegar House and passed it down to William in 1721 when he died.
William served as the Whig MP for Brecon and later for Monmouthshire, being returned for both in 1722, but he chose Monmouthshire. There was nearly always a Morgan as the MP for this constituency from the 1650s until the constituency was abolished in the 1880s, although one of the successor seats of South Monmouthshire was represented by a Morgan until 1906.
William married Rachel Cavendish, a daughter of the Duke of Devonshire, so that was handy at keeping power in the family. William was also one of the wealthy and enthusiastic Government supporters who was created a Knight of the Bath when that order was revived in 1725 and he was wearing his sash in the painting. William died young, although I’m not entirely sure what the cause of his death was as it doesn’t appear to have been recorded, but the estate was left to his son William Morgan the younger (1725-1763).
It’s not known who the painter was, but the National Trust note that on the artwork there is:
“Painted lower frame: Sir William Morgan, K B B.1700 D.1731 married Rachel, eldest daughter of William, 2nd Duke of Devonshire, KG
Written paper on crosspiece of stretcher frame: Sir William Morgan, Knight of the Bath, eldest son of John Morgan, esqr. of Tredegar. Belonged to Ruperra, 1783″
It doesn’t take much walking around Tredegar House to realise the power and influence of the Morgans on local and national politics, they had some considerable wealth going on here.