"Speakers of Welsh"
Why not "siaradwyr o Cymraeg"? To miss out the o looks like the speakers are Welsh rather than speakers of Welsh.
Cymraeg is both a noun (the Welsh language) and an adjective (Welsh-language, Welsh-speaking, etc). Cymreig is the adjective 'Welsh' when it is not concerning the language.
- siaradwyr Cymraeg - 'Welsh(-language) speakers' (or, more long-windedly, 'speakers of the Welsh language')
- siaradwyr Cymreig - 'Welsh speakers' (in the sense that they are Welsh, regardless of what languages they can speak.)
- afal Cymreig - a Welsh apple
- llyfr Cymraeg - a Welsh(-language) book
- llyfr Cymreig - a Welsh book (not necessarily in Welsh)
No since Welsh plural nouns are highly irregular. In this instance any word ending in "-wr" will become "-wyr", due to the "-wr" ending coming from "gwr" (Man/Husband, mostly Husband these days) which has the plural "Gwyr".
If you browse a dictionary and look at the plurals of words you will end up with something like 35-40 or more ways of creating plurals, depending on how you count them. You will spot some patterns, such as the one that EllisV has pointed out. There are almost always exceptions to the patterns, though, just to keep things interesting...
If you are learning your vocab using flash cards or a software app, it is worth looking up and noting down the plurals and also which nouns are feminine, too. That way, you can learn them as you go.
Sorry, not sure where to post this! I have just read a hint that says plurals of feminine nouns lose the effect of mutation. So if I say the girls is it y merched?