Regardless of whether the US Congress approves such acts of war, the US government has not been given approval by the UN Security Council for such attacks and has not even sought such approval. Such attacks are therefore in breach of international law, particularly the UN Charter.
The US will no doubt claim justification for such attacks as it did for its war against Afghanistan, and quite falsely for its war against Iraq in 2003. These wars were in contravention of the UN Charter, and the resultant deaths of hundreds of thousands of Iraqi and Afghan people were disproportionate to any threats to the United States. Ireland is also in breach of the UN Charter by facilitating the transit of US warplanes and armed US troops through Shannon airport.