Trump likes us, has interests here - and there's a hell of a lot of citizens in the USA of Irish ancestry, so not banning them plays well to his domestic audience. It is an election year after all.
On the other hand, Trump doesn't like the EU, and it's free movement principles. Not one little bit does he like the EU. The virus gives Trump a good opportunity to have a general swipe at the EU, and at the same time appear to be "doing the right thing" by keeping germ-carrying Europeans who could have travelled anywhere in the Schengen area, out of the US. If the infection rates don't get too astronomical, Trump will take the credit for taking tough action, and if it gets bad anyway he'll say "I tried my best but there were too many nasty foreigners already here". Trump doesn't care what the rest of the world, apart from his closest allies, think of his actions - and even then appealing to his own support base takes priority over everything and everyone else.