Pen-sarn and Tre-gynwer are settlements that developed primarily in the 20th and 21st century. They are located on the southern banks of the river Towy, whilst the town of Carmarthen is on the northern banks. The first edition of the 25inch OS maps, published in 1890, does not record Tre-gynwer (or Tre-gynwr) at all. Rather, it depicts Pen-sarn as a community of scattered cottages and farms. The settlement had a Calvinistic Methodist chapel - Capel y Babell (NPRN 6547) as well as a public house called the Queen's Arms, a smithy, a chemical works, a saw mill and a timber yard.
In the twenty-first century only Capel y Babell remains, but there have been substantial developments and the community now has several amenities. These include a primary school educating approximately 300 pupils through the medium of Welsh and English, as well as a high school called Ysgol Gyfun Gymraeg Bro Myrddin which educates approximately 900 pupils through the medium of Welsh. There is also a large retail park at Pensarn with shops such as Charlies, Aldi, Morrisons and McDonalds.
Sources: modern and historic OS maps, Google street view; Estyn report for Ysgol Gynradd Llangynnwr published in September 2017 and Estyn report for Ysgol Gyfun Gymraeg Bro Myrddin published in November 2015
M. Ryder, RCAHMW, 4th December 2018