Exporters of certain hazardous chemicals to non-EU countries have particular obligations (Export-Import Regulation (EU) No 649/2012 (further amended by Regulation (EU) 2018/172)). The objective is to allow countries to monitor and to control the export and import of certain hazardous chemicals. It also allows countries to refuse export and/or to place certain conditions on their import.
As of March 30 2019, the UK becomes a third country and so exporters of chemicals that are listed in the Annexes to the Export Import Regulation from Ireland to the UK will have obligations under the regulation (e.g. notification requirements).
Further details on the Export Import Regulation are available here while information regarding the post-UK withdrawal are available on ECHA’s website https://echa.europa.eu/advice-to-companies-q-as/pic