The experienced forward will lead the side in both encounters, scheduled for 24 and 31 October in Cape Town, following confirmation that regular captain Babalwa Latsha has been ruled out of the series with an ankle injury.
Solontsi is no stranger to leadership responsibilities, having co-captained the team alongside Unam Tose during their matches against Italy last month. Tose will serve as vice-captain for the two Tests, which mark the first time Ireland and South Africa meet in international rugby on South African soil.
The Springbok Women will also be without Nombuyekezo Mdliki, who has been unable to recover in time from a rib injury sustained against Spain earlier this month.
The replacements for Latsha and Mdliki are expected to be announced as the team finalises its preparations for the important home series.