score:5
The rule in the EU (which still applies for the UK until the end of the year) is that as long as the final airport has customs facilities, through-ticketed passengers will have their luggage checked through to the final destination, and will not see it at the first point of entry (contrary to what happens in the US).
I suppose she will go through passport control in London (not entirely sure what the rules are for transits through the UK to Ireland), but her luggage will be transferred to the next flight without her seeing it.
Her luggage will have a "regular" luggage tag (black and white) without the green stripes on the edges which are used for intra-EU travel.
In Dublin, when going through customers after retrieving her bags, she should NOT use the blue corridor (reserved for passengers originating in the EU, with green-striped luggage tags), but one of the usual green or red corridors.
Note that as this is related to customs, not immigration, the Common Travel Area is irrelevant anyway, only the EU customs union is. After the end of the transition period, of course, all bets are off on what could possibly happen and what the rules will be, though it is likely that in most of the possible outcomes, it wouldn't really change what would happen in this specific scenario.
Upvote:0
My cousin reports that her luggage was indeed checked through from SFO > DUB, and she didn't have to retrieve it at LHR.
OTOH, UK Immigration at LHR did ask her to produce her receipt for the checked bags.