The other day I heard another strange pronunciation of a placename by a BBC radio announcer who had evidently failed to consult the Pronunciation Unit or anyone else. It was Merthyr (Tydfil), which he pronounced as ˈmɜːðə rather than the usual ˈmɜːθə.
The English spelling th is completely ambiguous as between θ and ð in this position. On the one hand we have Arthur, McCarthy, Martha and Bertha with θ; on the other we have Carmarthen and ordinary-vocabulary words such as swarthy, worthy, northern, further and farther with ð. Some of us still remember the farthing ˈfɑːðɪŋ. With earthen(ware) some say one, some say the other.
Welsh spelling, in contrast, is unambiguous. Where the spelling is th the pronunciation is θ; where it is dd, it is ð. So if read as Welsh the spelling Merthyr can only signal θ. (Compare Caerfyrddin, the Welsh for Carmarthen.)
Although merthyr is the ordinary Welsh word for ‘martyr’, deriving via Latin martyr(-em) from Greek μάρτυς mártys (stem μάρτυρ- mártyr-) ‘witness’, it is also the Welsh form of Latin martyrium, from Greek μαρτύριον martýrion ‘testimony’, a word which came to be used to refer to a shrine consecrated with a saint’s bones. It is the latter that we have here and in other placenames: Merthyr Tydfil was named after the shrine of St Tudful. (It is a great pity that the Owen-Morgan Dictionary of the Place-names of Wales, from which I took this information, does not show pronunciations.)