Desmond is a fairly common English name, both as a surname and a given name. It is of Irish-Gaelic origin, from the place name of Deas-Mhumhna, or South Munster.
When the Kingdom of Munster split in the early 1100s, it gave rise to the Kingdom of Thomond (North Munster) under the O'Briens, the Kingdom of Desmond under the MacCarthys, and the Kingdom of Ormond (East Munster) under the O'Kennedys. The Ormond and Thomond names never really took off, but Desmond has remained a common surname and later a given name.
Desmond is currently the 369th most common boys name in America, sitting between Lawson and Winston; it peaked in popularity in 1992 at #265. It first started gaining popularity in America back in the early 1960s, presumably off the popularity of American jazz saxophonist Paul Desmond. It hit its first peak in the 1978, due to the influence of the South African anti-apartheid activist Desmond Tutu, and has remained a popular name among African-Americans ever since.
It is also a fairly common given name in Africa, particularly South Africa, and has a presence in most of the English-speaking world. One of the places that it is not very common, as a given name, is in the United Kingdom.