It can be done, and it has been done. The process runs through the warden's office and requires your fiancée to take the lead on initiating it from the inside.
The first step is for her to submit a formal petition to the warden requesting permission to marry while incarcerated. That petition needs to make a genuine case for why the marriage should happen now rather than after release. Wardens are not looking to approve these requests casually, so the petition matters. A compelling argument typically includes the length and seriousness of the relationship, the role the marriage would play in her reentry stability, and any other factors that speak to her character and her plans after release.
Several things work in her favor during that review. A clean disciplinary record inside is probably the most important. Wardens are far more likely to approve a marriage request for someone who has demonstrated they can follow the rules and conduct themselves well. Having served a meaningful portion of her sentence also helps, as does having the respect of both staff and other inmates, the kind of track record that signals she has used her time constructively.
If approved, the ceremony typically takes place in the visiting room. It is simple and brief, but it is legally recognized. You will need to handle the civil requirements on the outside, meaning obtaining a marriage license from Alameda County, where Dublin is located, before the ceremony can take place.
The facility chaplain is usually the best first point of contact for navigating the process. Your fiancée should speak with the chaplain and her case manager together to understand exactly what the petition needs to include and what timeline to expect for a decision.