2021 Dublin Bay South Bye-Election
Back-testing against a real bye-election
Dublin Bay South’s 2021 bye-election — called after Eoghan Murphy’s resignation and won comfortably by Labour’s Ivana Bacik. This is a further test our transfer model. The trasnfer system is designed for the present day, so the further back in time we go the less accurate it is likely to become.
Our Projection
Running Bacik and Geoghegan’s first preferences through our baseline transfer model has Bacik winning with 54.1% to Geoghegan’s 34.9% once everyone else is eliminated.
Bacik led from the first count and extended that lead steadily, picking up the bulk of transfers from Sinn Féin, the Greens, and the smaller left-wing candidates as they dropped out.
Results
Real Life
Bacik (Labour) defeated Fine Gael’s Geoghegan by 49.8% to 34.4% after all votes were counted. Sinn Féin’s Boylan and the Green’s Byrne were the last two eliminated before the final count, same as in our projection.
Bacik led from the first count here too, and built her winning margin gradually as the field narrowed, rather than in one late surge.
Results
Differences
The model got the shape of the race right — Bacik led throughout and won comfortably, with Boylan (Sinn Féin) and Byrne (Green) as the last two eliminated before the final count in both scenarios. The final margin is where it drifts: we had Bacik winning by 19.2 points, real life had it at 15.4.
With an updated set of baseline transfer rates, the model now also gets the order of the last few eliminations right — PBP’s Purcell goes before Aontú’s Tóibín in both our projection and reality, whereas an earlier version of the model had that pair flipped. With margins this tight between minor candidates, small differences in transfer assumptions are still enough to flip who’s eliminated in which order, even when it doesn’t change the final outcome.
We also ran one more count than the real result did. As with our Dublin Central back-test, the real count bundles minor and clearly-hopeless candidates into single eliminations where we process every candidate one at a time — it’s an unnecessary extra step for us since we’re not trying to speed up a live count, just get the transfer model right.
Reading the charts
Use the play button on each chart to step through the rounds, or use the round selector to jump directly to a specific count. The bar shading shows which candidates are still in contention; eliminated candidates drop to zero before their preferences flow to the remaining candidates.
These are illustrative scenarios, not predictions — first-preference shares and transfer patterns will both depend on who actually stands and how the campaign plays out.