Mostly I am following the possibility of going the other way as a citizen I have no problem getting back in but what qualification did they think they had for being able to get in with an ESTA. Looking at the standing NIEs for getting round the UK / Ireland travel proclamation it seems that the only ones that could apply would be US citizen / permanent resident spouse or unmarried child under 21. Individual exceptions may be being granted of course but you would expect these to have all been pre approved not for just rocking up with an ESTA.