Ireland can compete
According to the most recent 2016 world competitiveness survey Ireland is ranked 7th (up from 16th) and the US is 3rd (down from 1st).
There is no doubt that a reduced US corporate tax rate will encourage money back to the US from abroad and give those corporations more incentive to invest at home. As others have already noted; there is a huge cash stockpile waiting offshore for that to happen. Getting that money properly into the economy can only be beneficial.
However the president-elect is not just talking about lower corporate tax rates; he is also talking about massively protectionist trade policy and restrictions on importing a skilled workforce. Those things will lead to inflation on the consumer price index and to wage inflation, directly affecting competitiveness.
A protectionist US trade policy will make it all the more necessary for multinationals to have a presence in the EU.
In those circumstances I have no doubt that Ireland can continue to win direct investment and the gap in relative competitiveness will continue to reduce.