During the ratification of this treaty we in the United States Senate made clear that provisions of this treaty, and other[s] with the UK, should not be invoked pursuant to political goals related to Northern Ireland. In particular, the Senate resolution that accompanied the ratification of the extradition treaty in 2007 states that, "The Senate understand that the purpose of the treaty is to strengthen law enforcement cooperation between the United States and the [U]nited Kingdom by modernizing the extradition process for all serious offenses and that the treaty is not intended to reopen issues addressed in the Belfast Agreement, or to impede any further efforts to resolve conflicts in Northern Ireland.
Schumer has framed the question widely, addressing his concerns about the mutual legal assistance treaty between the US and the UK with a quote from the Senate resolution regarding a different treaty. But his argument is still specifically sound: A few years ago, the Senate ratified a treaty between these two nations regarding a matter of international cooperation in internal criminal justice matters. Doing so, they make explicit their intent to keep "issues addressed in the Belfast Agreement" -- like the past activities of paramilitaries that fought during the Troubles in Northern Ireland -- out of the bucket of things for which the treaty would assure police cooperation. So why would the same United States Senate ratify a different treaty, just a few years earlier, with an entirely different intent?