I hope I can shed some light on this for you.
As you commented, Waunystradfiris is noted on the 1881 census as on ‘District Road’. I then looked for other properties also listed along ‘District Road’, and came up with quite a list for my trouble! Armed with this information I cross-referenced this with a very useful mapping service available on the Carmarthenshire County Council website (http://gis.carmarthenshire.gov.uk/planaccess/planaccess.asp
), which is searchable by address or part thereof. Although I could not find Waunystradferis, there is a Waunystrad-meirig and a Meiris Villa. The others I found that matched the 1881 census were; Mount Dyrfal, Cwrywaun, Tynlone, Tynlonefach, Greenfield Cottage, Pantywaun, Penybont, Maesgwastod, Llwyncelyn. All of these were fairly close to each other, so I’d have to say, we must be pretty close. If you want to Multimap or Old maps, the post code is SA19 9HS. District Road appears to correspond with the C2149 from Llangadog to Llandovery.
My best guess would be that the house is either not there now, or the name has been changed, or even that the census enumerator miss-spelt it (unheard of eh!!). I’ve not been along that road for a while but I’ll be going for a nose when I’m passing next.
Hope this is some help.