An tan is caeine an cluiti…
An tan is caeine an cluiti is and dlegur a discur.
(the * time * is * more beautiful * the * game * is * then * it is proper * its * ceasing)
The best time to give up the game is when it's at its best.
This maxim is found in “Lorgaireacht an tSoidhigh Naomhtha”, the 15th century Irish version of “La Queste del Saint Graal” (The Quest for the Holy Grail). A more standard spelling of the sentence would be “An tan is caoine an cluiche is and dlegar a díscor.” “Quit when you're ahead” may come close to it in today's English. In Modern Irish I'd say “Nuair is fearr an cluiche, is ansan ba chóir éirí as.”