My understanding is that a person living in the UK on the basis of being a spouse/partner of a holder of a UK Ancestry Visa is entitled to work. (Note, though, that the granting of the visa in the first place – to the main visa holder – is dependent either on that person intending to seek and take employment, or on that person having a job to go to.)

I am not sure, however, of the need to travel with the main visa holder. Certainly that is not the case once everyone has arrived (i.e. the spouse can travel abroad and return to the UK without the need for the main visa holder being present with him or her on the trip). This may be true as well for the first entry – I don't know – but if it is there might be need for proof to be shown that the main visa holder will be travelling to the UK forthwith. Certainly the subsidiary visa holders can enter for the first time after the holder of the main visa (which is a very different thing, of course). The best way for the OP to clarify this would be to check with the British High Commission in South Africa, or to seek the advice of a competent lawyer who knows about British immigration matters (and there must be some in South Africa). What does the visa in your passport actually say?

Also to the OP: there is no point in trying to "sneak" into the UK via Ireland or via some regional British airport. If you have the right to do what you propose to do – which you need to clarify one way or the other – then there is no need to. If you do not have that right, then the Irish will be on to it (they, after all, will see the visa in your passport, and moreover they are very alert now to people trying to use Ireland as a "convenient" way to enter the UK), as will the immigration officers at any British airport.

Good luck with finding out what you need to.