Well the best way to know for sure would be to give it a try. 8 culture per birth can be a lot in the early game so your start should be decent. I'm just worried about happiness.
Ignoring the 25%

Growth vs 1


on all resources. (Rhiannon is better)
Bran gives 8

per birth. Rhiannon gives 8

every 4 turns (and contributes to boredom reduction)
Rhiannon's actually middle of the road for Celtic Pantheons, and it beats Bran's numbers outright. Even Goddess of Love, the base Pantheon, gives 15

/10

/5

. That's uncomfortably good in comparison to a Celtic pantheon's 8

.
Bran also has the dubious honor of being the only pantheon that gives less than 3 yield types (if you consider Ogma's

GSP a yield)
Seems like an easy number's tweak to get Bran up to a respectable level; maybe 10

/

on birth and 33%

growth?
Some other notes:
- Rhiannon is the only one who has “goddess of” in her name and it sticks out like a sore thumb. Could we not call her “Rhiannon the Sovereign”?
- While on the subject, Morrigan’s “the harbinger of strife” is a bit of a mouthful. Maybe just “the Harbinger”, or the “The Three in One”, or maybe “the Phantom Queen”?
- cernunnos needs a bonus to plantation. The forest yields make clearing a forest plantation resource at best a value-neutral proposition. Maybe 1

to match the camps?
- Bran, Morrigan, and Epona are the only beliefs that don't help with happiness at all (instant yields only)
- Weird that Epona gives no bonus to border growth while God of the Expanse does. Maybe Epona should give 3 Border Growth Points in all cities, and then another 3 BGP at Ceilidh hall? 4

alone on Ceilidh is easily the weakest Ceilidh boost of the lot.
- Morrigan gives 100% of a unit's

CS as

/

/

. I think this is a good level, because you don't want it to out-Aztec the Aztecs (150%

/

on kills), but it's not quite enough on its own. I wonder if maybe Morrigan could give new units 10XP or something?
- Goddess of Beauty gives

GArtist and GEng points, but Ogma only gives 1 type: GScientist. Shouldn’t ogma, the god who invented the Celtic script, give GWriter points? I guess you wouldn’t have anywhere to put the GW for your first writer, but it still seems off.
- I don’t agree with
@CrazyG's assessment of Lugh. Getting a specialist in each of your cities forces you to rush trade for markets ASAP, on the opposite side of the tree from Pictish warrior. Then you have to build the market. Then you have to get enough

food and

population to actually fill the slot. It gives no food to support that specialist, so you’re forced to support a -3

food negative citizen earlier than you reasonably would want to. Compare that to Mannan, who gives only slightly worse yields with no setup whatsoever. Lugh should have its 3

gold replaced with 4

food. Without some way to support an early specialist, this belief is downright masochistic.
- Mannan and God of the Sea both give

food to coastal cities, and this is the only pantheon I see that gives the same bonus as the comparable base pantheon (besides dagda/god-king, but that’s unavoidable). Maybe swap the

food for

science?