Ireland provides an interesting setting for the study of incumbency advantage. Its electoral system creates incentives for political candidates to cultivate a loyal, personal following and the rate of incumbent re-election is one of the highest in the world. This paper exploits the quasi-experimental features of the system of proportional representation with a single transferable vote (PR-STV) to estimate incumbency advantage in Ireland’s lower house of parliament. In very close elections, where there is a narrow margin of victory, it is likely that bare winners are comparable in their unobservable characteristics to bare losers. Regression discontinuity design (RDD) identifies the causal effect of incumbency by comparing the subsequent electoral outcomes of bare winners and losers. The analysis indicates that incumbency causes an eighteen percentage point increase in the probability that a candidate is successful in a subsequent election. We show that Ireland’s multi-party, multi-candidate system is particularly suited to the application of the RDD methodology.