I have to admit I found CITY Dulas by using the OS map, something rang a bell about a City in Anglesey, some trivia from years ago I think! On the map it does show a monument (mon) so I went onto Google maps and could see it from road level, that gave the name Moruslaid, but I could not get any sense from that ... no wonder, as Google maps has spelt it incorrectly - nothing new there! Morrisiaid (spelling on the plaque) translates as Morris's, the family surname.
Zooming in on your photo did not work, could not read the inscription, so thanks for the detail - someone on here must be able to translate it fully but it does seem, from my limited Welsh, that there were three brothers, Lewis, Richard and William, back in the 1700's and they were involved in Welsh literature, poetry and music, and also the Welsh bible. For some reason Richard is buried in London. The memorial has been there since September 1910.