As a Norwegian learner of Welsh who has never lived in the UK, I have never heard neither the English nor the Welsh names of quite a few of these towns outside of Duolingo :-)
What did shrews bury??
Can Dw i'n also be written Dwi'n?
That is how it is pronounced, and so some people do write it like that. If you keep them separate when writing, though, it will remind you that the dw is the verb '(I) am' and the i here means 'I', the pronoun.