Tough call.
It has to be a group song with all of them.
I of course am madly in love with My Land. And it would allow both Mairead and Tara to be in the song (Tara on harp, like in Destiny):
https://www.youtube.com/watch?v=wOee2M_WoxQHowever, My Land doesn't give a solo part to each of the singers.
I also love Westering Home. It has slightly more pace than My Land and gives a solo part to each singer.
Óró Sé Do Bheatha 'Bhaile is a good choice from a cultural standpoint (Easter Rising Centernary).
Bean Phaidin kind of has the best of all worlds for a general audience. It gives everyone a solo, it has moderate pace. It has Irish and English and it's a fun song too. I can see a general audience getting into this one and clapping along. One might argue the harmony is less spectacular than in the previous 3 songs mentioned though and personally, I think their harmony is their biggest draw.
For the moment, I'm going to go with Westering Home for it's harmony and because it has solo parts for all the ladies. But there are lots of good choices.