The name of the confluence of the river "Monnow" with the Wye [Aber-Mynwy] became the mouth [aber] of the Monnow, translated in the English Mon-mouth. First the site, then the town, and then the county it became.
The first of the Jones surname was Hugh "Jonys", 1538 - 1544. [bundle 1016/ no. 59] A Maude Jevan, daughter and heir to Jevan ap Griffith (1504 -1515); a Jenkin ap Jevan, carpenter (1538 - 1544); and Edward Jeyn, yoeman (1538 - 1544) were also found. Hugh Jonys was identified as "groom of the King's Chamber, and nephew and heir of Edward "Davyd" clerk. The spelling of "Jones" begins with a David Jones (1547 - 1551) [bundle 1238/no. 38] which again shows the transition from "Jevan" to "Jonys", to "Jones".
A discussion of Monmouthshire can be found in Nicholas, Vol. II, pp. 715 - 787.