According to some information (unverified) that I found on the web, this Philip Monckton was the son of Edward Monckton (son of John Monckton, 1st Viscount Galway) and Sophia Pigot (illegitimate daughter of George Pigot, 1st Baron Pigot). Interestingly, the 8th Viscount Galway was Governor-General of New Zealand from 1935 to 1941.