I would say là éigin . And the free form is subjectless, so gar a dhéanamh might be more likely. I would be tempted to remove agat as you already have ort to call the favour (i assume you do not intend to ask...)
Toask a favour of s.o., gar a iarraidh ar dhuine. Todo s.o. a favour, soilíos m, gar m, a dhéanamh dodhuine. Will you do me a favour?an ndéanfágar dom?