This is cool You don't subscribe to James Holloway's patreon, do you? His Patron Deities podcast has some really interesting stuff on using Irish mythos in RPGs.
Truth be told, haven't even heard of him let alone his Patreon.
Onto the questions.
How do social elites interact with faith? Do they donate publicly to keep groves, or ostentatiously turn up in a huge car to leave massive oversize offerings? Or if there's some mechanism that counters that, what is it? (Maybe for example they competitively build private chapels to show off to each other and consider the "common" temples as kind of vulgar).Donations; depends really on the family/person.
Some would be more devout and regularly contribute towards upkeep of chapels and groves, others may make the odd and sizeable donation for publicity sake.
As for private chapels, they tend to be built if a particular family favours a certain deity. For example, a family/clan with a strong warrior/military tradition may have chapels devoted to Morrigan.
What social class do keepers tend to be? Do elites look down on them or up to them/what level of respect-for-office do they get?Keepers generally come from a middle or working class background, those from the upper class aren't unheard even if they tend to be the second or third born child of the family.
The Keepers' standing is probably similar to that of a parish vicar, at least respected for tending to the people's spiritual needs. Though families aligned with a particular deity are more respectful of Keepers who are aligned with the same deity.
Course, if a social elite is disrespectful to the Keepers they could find the people being rallied against them.
There are also Keepers who served or do serve in a semi-military capacity as either medics or battle chaplains, these probably receive the greatest respect....then again it's hard to disrespect a crazy priest running around with a battleaxe.
What kinds of ritual are performed where? Like where and how would you perform a marriage or a funeral? Are there civic occasion rituals, feasts, or offerings, and who gets to perform them?Marriages aren't that much different from our own, obviously incorporate more Celtic style traditions like hand-fasting and the like. Traditionally held in a chapel or grove, but nothing says these have to be the only places.
Funerals are different, being week long affairs. Keepers (typically low ranking ones) would wash the deceased and then stay with them, burning candles night and day. A more senior Keeper will lead a wake a few days before the body is interned or cremated. At sunset on the final day is the actual burial or internment takes place. Though there are extreme circumstances where the funeral may take place in a shorter period.
Festivals are basic the Celtic festivals, along with days throughout the year devoted to a certain deity. The important the ceremony or service, the more senior ranking Keeper leads but those of a lower rank may occasionally be given the 'honour' to head a particular ceremony.