The outcome of by-elections shows no meaningful incumbency advantage: the party holding the seat prior to vacancy retained it in 51.45% of cases (71 times), essentially the same as the proportion in which the seat changed hands. In other words, by-elections do not systematically favour continuity over change. Results are therefore almost evenly split between retention and loss, indicating that the previous party’s hold on the seat provides only a marginal predictor of the eventual outcome. While we can get an understanding of incumbent performance through the last dataset, a further analysis of the figures allows us a deeper insight into the success of both the government and opposition at the time of a by-election. 

The by-election data show a roughly balanced distribution between outcomes favoring the government and those favoring the opposition. The government successfully retained or gained seats in 46.38% of by-elections, while the opposition held or captured seats in 53.62%. Individually, the most common outcome was Opposition Hold (34.78%), followed by Government Hold (26.81%), suggesting that incumbents, and particularly opposition incumbents, tend to retain their seats more often than they are lost. Overall, while the government performs moderately well, by-elections slightly favour the opposition, reflecting their potential as a mechanism for signalling voter dissatisfaction or shifting parliamentary balance mid-term.

Turning to the causes of by-elections, this dataset shows that the overwhelming majority of by-elections were triggered by the death of a sitting TD, which accounts for 67.4% of cases. This indicates that natural attrition has historically been the dominant cause of mid-term vacancies in Dáil Éireann. Resignations are the next most frequent single cause, with 18.12% instances, reflecting voluntary departures for political, personal, or strategic reasons. Other causes are comparatively rare but demonstrate the diverse procedural and institutional factors that can trigger a vacancy. Disqualifications account for only 3 cases, showing that legal or eligibility issues are an infrequent cause of by-elections. Multiple-office rules also contributed a small number of vacancies, captured under the category “Incompatible office”, which totals 14 cases and includes appointments or elections that make a TD’s Dáil seat incompatible with a new role.