Likely the Heraia in Argos, since that was the finding place of the source.
The SEG no. (11.338) is not available on PHI. The source mentions various victories which can tentatively be dated around 200 BC, cf. note 341 in Farrington (2012), based on Moretti IAG 45 - Farrington's dating has been followed here. The name is no longer extant, nor is the athlete's origin - Farrington followed Moretti in designating him Achaean, which is suggested by line 7 of the inscription, and Argos, the finding spot of the inscription, was part of the Achaean League around the time to which the source is dated. The source gives various other events in which the athlete participated, apart from the Isthmian Games. It is said of his Isthmian victory that he won 'pentakis'. - PK