Irish Water has agreed a plan with its Northern Ireland counterpart to bail out border counties with tankers and pallets of bottled water if a hard Brexit leads to shortages.
Internal documents show the state utility has been engaging with Northern Ireland Water on a "reciprocal support and aid agreement".
Under the arrangement, both bodies agree to support each other in the event that a crash-out Brexit impacts water supply on the island.
Emails and documents seen by the Herald show the aid could be necessary in particular for Irish Water customers on the border whose supply is delivered by Northern Ireland Water.
Internal UK government documents have previously warned that some of its fresh water supplies could be hit if there are problems importing water treatment chemicals.
Irish Water has noted in its internal documents that 500 residential customers are supplied by three different cross-border supplies for which Irish Water pays its Northern counterpart.
"This aid will apply on an all- island basis and will be of key support to our border counties customers," an Irish Water official wrote in an email in January.
"Especially in the event of supply being impacted to those Irish Water customers that are served by an NI Water supply."
A source said this could include providing alternative supplies, including water tankers or pallets of bottled water, to customers hit by shortages.
Irish Water said: "Mutual aid in an emergency might include provision of expertise or personnel; IBCs (Intermediate Bulk Containers) or similar, if an alternative water supply was required; or possibly engineering parts."
Des Nevin, of Northern Ireland Water, said: "Our customers should feel reassured that we are prepared and arr- angements are in place for minimising risk."