Jean,
Firstly I must apologise as I made a mistake. The Valuation Revision records (on the PRONI website) show the farm going from Thomas Mitchell to George Thompson, not to George Mitchell. Sent you off on a wild goose chase there. George Thompson died 26.5.1884 at Camnish aged 80. His widow Jane was the informant. Their daughter Mary Ann Thompson married William McCloskey on 8.11.1898. There could well be a connection with Agnes McCluskey who married Thomas Mitchell. Cousins often married each other, especially in farming circles as it helped keep the land in the family.
I looked for an Agnes Mitchell death 1864 - 1874 but did not see one. So possibly she died before death registration started in 1864, in which case there may be no documentary record. She might be on a gravestone of course. What Thomas Mitchell was leasing to George Thompson was a labourer's cottage (without land). Often there was no connection between a farm owner and his labourers but in this case since George inherited the farm, I think it likely that there was a connection. As you say, George could have married one of Thomas's daughters. There was quite an age gap there. According to the 1901 census Jane was 70 (so born c 1830) but George was born c 1804 (though ages on death certificates are notoriously unreliable. Often just guesses). They probably married in the 1850s. If the farm went to George Thompson that also suggests there was no Mitchell son able or willing to take it on.
Hope you can make your trip next year. Nobody really knows what's going to happen and few people are booking long distance holidays at the moment. Ireland hasn't been hit too badly compared with some countries but there are still cases about and not everything has re-opened. (PRONI, for example, remains closed). And there is 14 days mandatory quarantine for folk arriving from outside the British Isles, so that makes tourism pretty pointless at the moment.