This is according to Irish Rail's service report for April to June, which was released today.
The Heuston to Kent train scored 91.2% in punctuality performance for the second quarter of the year.
The same service had an overall punctuality score of 90.7% for the year to date, again the lowest in Ireland.
However, the score was still quite high, and just beat the 90% set target.
Passengers travelling via train within Cork county experienced more punctual services.
The punctuality score for the Cobh line was 99.5%, Midleton received a near-perfect 99.9% score, and Mallow got a 99.7% score from April to June.
From January to June these Cork county routes all received over 99% in their punctuality scores.