It is the dialect of Okinawa, Japan, and has no special meaning. The main thing is to let people who sing and dance together find the same beat.

Just like when two or more people carry things in Japanese work (ユーショー), it is similar to our slogan (1, 2,3).

I ya sa sa is basically a slogan used by Okinawa people when they play drums and dance together.

The same slogans are "はーぃや" and "ェィサー".

As for "ェィサー", it is the beat (slogan lyrics) mixed in the chanting songs of Pure Land Sect.

Of course, the song you said should be Okinawa's song. In order to show its characteristics and regionality, some Okinawa lyrics have ぃやささ lines. But usually it's not the lead singer, but the chorus.