I have been asked to reply.
There is no evidence that the US represents a significant source of funding or other support for dissident Republicans. In terms of prevention, both RIRA and CIRA have been designated 'Foreign Terrorist Organisations' by the US Government. This makes it unlawful for a person in the US or subject to the jurisdiction of the US to knowingly provide material support or resources to a designated foreign terrorist organisation. It also requires any US financial institution that becomes aware that it has possession of or control over funds in which a designated foreign terrorist organisation or its agent has an interest, to retain possession of or control over the funds and report the funds to the Office of Foreign Assets Control of the US Department of Treasury.