Brexit could "wipe out" towns and villages along the Irish border, a parliamentary committee has heard.
Niamh Smyth, a TD (MP) for Cavan and Monaghan, counties on the southern side, said unrestricted cross-border trade was critical to the recently improved fortunes of the once bady-deprived region.
Pointing out the reliance on the dominant agri-food industry in particular, she said many dairies and food businesses operated as if no border existed.
"As we know, various stages of production happen on both sides of the border, there are gallons and gallons of milk going from one side to the other, and as it stands at the moment you wouldn't even know you are crossing the border," she said.
"If that were to change, to become a very visible, tangible, hard border, who knows what that would mean for these industries... it could wipe out a constituency, it could wipe out the whole border region."
Ms Smyth told Dublin's Arts, Heritage, Regional, Rural and Gaeltacht Affairs Committee, that it was "soul destroying" to contemplate the border region returning to the dark days of the Troubles-era checkpoints and a hard frontier.
"We have come so far, it is unrecognisable going back to when I was a child," she told the committee, investigating the impact of Brexit on the region.
"Nobody lived in these towns, never mind worked in them, because there was no purpose to them, there was nothing to keep us there.
"They really are only getting on their feet. There is still a long way to go in certain towns and villages, and this could just wipe us out."
Ms Smyth added: "There has to be a special case made for the border."
JJ O'Hara, a tourism official from the Republic who is in the Border Communities Against Brexit organisation, told the committee the reality was that a renewed hard border will "bring trouble".
Any new UK-EU frontier border should be "in the Irish Sea and not in Ireland", he said.
Mr O'Hara added: "The reality is that we are a very small island, and to be cut in two is not an option."
John Sheridan, a farmer in Northern Ireland who is also part of Border Communities Against Brexit, warned against Brexit negotiations being allowed to "ruin" Ireland.
"I really believe, and I have lived on the border all my life and spent probably as much time in the south as I lived in the north, it would be a shame to let Brexit, which is triggered today, ruin this country," he said.
Separately, Ireland's Small Firms Association (SFA) has told businesses to "prepare for the worst".
Sue O'Neill, SFA chairwoman, said: "Issues affecting the border with Northern Ireland and businesses who operate close to the border must be given the attention and sensitivity that they deserve."
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