Anwen is one of the simplest and best of the classic Welsh girls' names, more unusual than Bronwen but with the same serene feel. 

Anwen

This early Welsh saint's name is unusual and lovely, waiting to be discovered.  Saint Arianell was a member of the Welsh royal family who was possessed by a spirit which was exorcised by Saint Dyfrig, who became Arianell's spiritual teacher.

Arianell

Very popular in Wales, this perfectly nice name's only problem is its similarity to the dated Beth Ann. 

Bethan

Unusual floral name which would be perfect for a spring baby girl, if you can pronounce the Welsh double L correctly. It's a tricky sound for non-native Welsh speakers, kind of halfway between an H and an L.

Briallen

Celtic goddess of poetry, though less-than-poetic name. 

Ceridwen

Common name in Wales that's all but unknown in the U.S.  Certainly an attractive choice ripe for export. In the UK it sits at Number 330. 

Cerys

This Welsh nature name that can be pronounced exactly like the male Ira. Or, to avoid confusion, you could just name her Snow. 

Eira

Evanna is the name of a Harry Potter actress who pronounces it ee-vah-na. This might be considered yet another international feminine form of the many variations of John; Ivana, Giovanna, and even Joan are cousins of Evanna.

Evanna

Guinevere was the name of the beautiful but ill-fated queen of Camelot, for so many years eclipsed by its modern Cornish form Jennifer. Today, Guinevere could be a cool possibility for adventurous parents intrigued by this richly evocative and romantic choice.
 Guinevere shares the attractive short form Gwyn with Gwyneth. Other cousin ideas: Gwendolen, Genevieve, Jenna, or Gwen.

Guinevere

Now that Tristan has been rediscovered, maybe it's time for his fabled lover in the Arthurian romances and Wagnerian opera, a beautiful Irish princess, to be brought back into the light as well. 
 This would make an artistic and romantic choice, as the medieval heroine is seen as a symbol of undying love. 
 In Marilyn French's novel The Women's Room, the character Isolde is nicknamed Iso. Isolde is currently a Top 500 name in the Netherlands. Variations include Iseult, Isolt, Isoud, Isolda and Yseult.

Isolde

Megan originally evolved from Meg, which itself derived as a nickname for Margaret. Margaret ultimately comes from the Greek word margarites, meaning "pearl." Megan is no longer a common nickname for Margaret—it is most often used as a full name. Other spellings include Meghan, Meagan, Megyn, and Meaghan.
 
One of the most popular <a href="https://nameberry.com/baby-names/172/welsh-names">Welsh names</a> ever in the US—it was Number 10 in the 1990s—Megan is still widely used today, but waning. It fell to its lowest ranking in 50 years in 2017. Actress Megan Fox has upped its glam factor. We've yet to see how the popularity of the new Duchess of Sussex, Meghan Markle, affects all the spellings of her name.

Megan

Meredith is a soft, gentle-sounding name with subtle Welsh roots. Although originally a <a href="https://nameberry.com/boys-names">boys’ name</a>
, Meredith is used mainly for girls now.
 Meredith's highest point of popularity for girls in the U.S. was in the late seventies and into the eighties, when it ranked as high as Number 140.
 Meredith, when said with the American and English pronunciation, offers the delightful "Merry/Merri" as a nickname. Be aware, however, that the correct Welsh pronunciation has the emphasis on the second syllable (muh-RED-ith), making "Reddy" a more suitable nickname.

Meredith

Morgan has long been a traditional Welsh male name, a variant of the Old Welsh name Morcant, from the Welsh elements mor, meaning "sea" and cant, "circle." The female Morgan is unrelated to the male version—it is a name from Arthurian legend created for Morgan le Fay, King Arthur's half-sister and famed sorceress. Her name comes from Morgen, an Old Welsh name meaning "sea-born," and is related to the Irish name Muirgen. 
 
In this country, Morgan is now used more for girls than boys, though considerably lower than its Top 25 status in the 1990's. And although it's been veering south for both sexes, we still find Morgan a sophisticated choice. 
 
Actress Morgan Fairchild—who had an influence on the name's popularity for girls—was born Patsy.

Morgan

American parents are just becoming aware of <a href="https://nameberry.com/baby-names/172/welsh-names">Welsh names</a> like this one, similar to Carys, which was chosen by Welsh-born Catherine Zeta-Jones.

Nerys

Penrose – an ancient place-name and surname derived from several villages in Cornwall, Wales, and the Welsh border country of England – would make for an even more unexpected alternative to quirky nature name Primrose. Intuitive nicknames Penny or Rosie make it feel more wearable.

Penrose

The sassy, steel magnolia appeal of Oscar-winning Reese (born Laura Jeanne—Reese is her mother's maiden name) Witherspoon has single-handedly propelled this formerly <a href="https://nameberry.com/boys-names">boys’ name</a> into girls' popularity.
 Reese was recently in the Top 10 Unisex names looked at on Nameberry.

Reese

Popular in Wales, but likely to be mispronounced as Ryan here. 

Rhian

Most of us had never heard this lovely Welsh name with links to the moon until we heard the 1976 smash hit Fleetwood Mac song of that name, with lyrics by Stevie Nicks. That same year it popped onto the U.S. Top 1000 at Number 593. 
 After giving his four boys extreme rebel names, director Robert Rodriguez bestowed this sweet name on his first daughter.
 According to Celtic legend, Rhiannon's birds sang more sweetly than any others; she herself was known for being forthright and witty.
 Rihanna looks similar but is unrelated.

Rhiannon

The delicate and haunting Welsh Rhonwen is still a rarity in the U.S., where her English version Rowena is better known, but would be a lovely choice for any parent in search of a name that was both unusual and traditional, classically feminine yet strong.
 Though <a href="https://nameberry.com/baby-names/168/irish-names">Irish names</a> have long been popular in America, <a href="https://nameberry.com/baby-names/172/welsh-names">Welsh names</a> are just beginning to be noticed and we see them as a wave of the future. In medieval Welsh poetry, Rhonwen is called "the Mother of the English Nation," and Rhonwen Baganes was a heroine of Welsh folklore. A soft and gentle name, Rhonwen's meaning -- slender and fair -- evokes the image of a lovely, light-haired girl. Other appealing 'wen' choices include Aelwen, Arianwen, Branwen, Bronwen. Gwendolen, Olwen and Tanwen. Just don't be tempted to change the 'wen' to 'wyn', which would transform it into a male name.

Rhonwen

Seren is a top girls' name in Wales – and a lovely choice almost unknown elsewhere. Seren, in the Sirona form, was an ancient goddess of the hot springs.
 Seren only recently fell out of the girls' Top 10 in Wales, and ranks just below the Top 100 in England and Wales combined.
Seren is also a Turkish surname and girls' name.

Seren

Taliesin is a name an architecture-minded parent could love, because of its close association with Frank Lloyd Wright. 

Taliesin

Tegwen is nowhere near as pretty as its meaning. Other -wen names -- Anwen, Bronwen, Rhonwen -- have more appeal.

Tegwen

lottalove99's list "Melodic, Flowing Welsh Girl's names" of 22 great name ideas: Anwen - Tegwen!

Melodic, Flowing Welsh Girl's names

I just love these beautiful, melodic Welsh names which are easy to pronounce and many have lots of possible nicknames ready to use!