The British did declare was on the Dutch during the American War of Indpeendence:
https://en.wikipedia.org/wiki/Fourth_Anglo-Dutch_War
Here is the quote from that article in reference to the Cape Colony:
"In March 1781 British admiral
George Johnstone was sent to capture the Cape Colony. France, which had already planned to send a fleet to India, received intelligence of this, and directed its commander, the
Bailli de Suffren, to try to reach the Cape before Johnstone. After Johnstone and Suffren met in a happenstance
battle in the Cape Verde Islands, Suffren was able to arrive before Johnstone, and the strength of French troops he left dissuaded Johnstone from attacking the colony. After capturing a number of VOC ships in the nearby
Saldanha Bay, he returned to North Atlantic waters."
With that out of the way, diverting the stream of convicts from Australia to the Cape has huge effects later in history. One question, which I don't know the answer to, is how much the British bother with Australia.
Note that the Bantu did not go past the Great Fish River when they migrated into what is now South Africa. The Western Cape is the one province in South Africa where Bantu/ Africans are in the minority (33%) and this is after many were brought in for the cheap labor. With lots of Irish convicts available, there would be no need for the cheap labor, so a much greater Colored (IOTL close to a majority) and White proportion of the population. At the same time, the area would be much less Afrikaner in culture and language, and considerably more Catholic.
Basically replace the current demographics of the Western Cape with that of New South Wales, plus a higher population. Other parts of South Africa gain more Bantu and probably more Afrikaners as well, as more leave on the Great Trek.
There is a good chance that this butterflies into no Union of South Africa, and if there is a Union of South Africa there will be much more support for the Labour Party in the (whites only) early twentieth century elections, and much less for the various Afrikaner nationalist parties. With the Union of South Africa not being formed or likely to fall apart, the British would have to come up with some other scheme to maintain control of the Transvaal mines.