In 1870-72, John Marius Wilson's Imperial Gazetteer of England and Wales described Undy like this:
UNDY, a parish in Chepstow district, Monmouth; on the South Wales railway and on the coast, 1 mile E of Magor r. station, and 7¼ WSW of Chepstow. Post town, Magor, under Chepstow. Acres, 3,717; of which 2,000 are water. Real property, £2,653. Pop., 411. Houses, 78. The property is divided among a few. The living is a vicarage in the diocese of Llandaff. Value, £183.* Patrons, the Dean and Chapter of L. The church is good.
GB Historical GIS / University of Portsmouth, History of Undy in Monmouthshire | Map and description, A Vision of Britain through Time.
Date accessed: 25th March 2017
Click here for more detailed advice on finding places within A Vision of Britain through Time , and maybe some references to other places called " Undy ".