The terms ap (or "ab") and ferch (or "verch") are Welsh terms meaning "son of" or "daughter of," respectively, as in Madog ap Rhys and Gwenllian ferch Rhys.
Personally, I would use the "ap Madoc Vychan" that is used for Lloyd_ap_Madoc_Vychan-1 But, then I am used to the patronymic surnames of my Dutch Ancestors. It may be different for the Welsh, but we still don't enter the "of...place name" in the LNAB field.
You (or Antonya) may wish to defer to the Policy and Style gurus that she tagged in her original question. Also, the name appears on pages 838 and 858 of this book: https://archive.org/details/cu31924091752034/page/n895/mode/1up/search/LLwyn+dyrys