The name Celyn is a lovely name for a girl - but traditionally it's a boy's name in Wales (although not a common one) as well as a place name. Just as the name Meredith (in Welsh it's spelled Meredudd) is a boy's name in Wales, but is nowadays more commonly used for girls. And since Celyn is also a noun meaning holly, which is in English a girl's name, there is a logic to it.
My son is called Celyn (yes - it's correctly pronounced Kelin) and suffers from the rise to fame of CelineDion -- people assume it's the same given name, especially as we live in Canada!
In Welsh the letter C is always pronounced K, and there is no letter K in the Welsh alphabet. Welsh often has the appearance of having no vowels (we're too poor to afford them!) but in fact the letter Y is also a vowel, which makes for interesting editions of Sesame Street!
And there is another Welsh name that in English looks like it should be pronounced like Celyn - it's 'Cellan' and is both a boy's name and a place name. Trust me - the pronunciation is not the same, but the sound represented by "ll" in Welsh does not occur in English so I don't know how to explain it!
One last thing, the name Celyn is a bit of a mystery -- it appears as a mans name in the Welsh medieval tale 'Culwch and Olwen' (see The Mabinogion) but I've been told that the use of the word celyn for holly is a more modern practice - ie the word was used for a name before it became used as a noun. I have no idea what they used to call holly in mediaeval Wales!
With regards to the request for flower and place names in Welsh - that would be rather a long list and difficult to pronounce!! Some flower names used as girls names include Blodeuwedd - (flower-like) Blod-oi-weth Blodwen - (beautiful flower)Blod-wen Eirlys - (snowdrop) Eye-rl-iss Ffion - (foxglove) Fff -ee-on Fflur - (flowers - this word is really the same as the french Fleur , spelled in Welsh and with similar pronunciation to the French word) Gwenydd - (Wheat -- not a flower I know, but a pretty name along botanical lines!) Gwen-ith (sometimes spelled Gwenith) Lili - (Lilly - and pronounced like that too.)