I would fly United Dublin to San Diego, transiting in Newark. Transfers in the US are generally ok if you have a single PNR (ticket) and not changing airline. Plus EWR is a United hub, so if there are any aircraft issues they tend to have standby aircraft available. The United section of EWR is well laid out with a lot of facilities and is easy to pass time in, and the lounges are also very good. Because of Dublin Pre Clearance, you land in the United domestic area so your onward gate is in the same or an adjacent section, and may be just a few gates away. Only downside is the DUB-EWR leg is more often than not a narrow body aircraft which some people don't like.
I have used United and Newark a lot when flying further afield in the US and I've never had a problem. Far preferable to say an Aer Lingus/Jet Blue mix, which can get you a single PNR but the interface between the airlines and their systems is woeful. Transfers between different airlines booked with separate tickets is to be avoided, as any problems tend to be entirely yours at the end of the day.
I'd walk rather than face into LHR on the way home.