Llinos Medi, Plaid Cymru MP for Ynys Môn, is urging the UK Government to extend the transition period for the new requirements, which take effect on September 28.
Passengers using affected ferry routes between the Republic of Ireland and Great Britain will be required to present a valid passport or Irish passport card, with all other forms of identification no longer accepted.
Ms Medi said: “This change has been announced with just over a month before implementation.
“While I recognise the importance of maintaining secure and effective border arrangements, the timescale for adaptation to this new rule is simply too short for many people and businesses who regularly travel between Great Britain and Ireland.
“Many frequent travellers have historically relied on alternative forms of identification when travelling across the Irish Sea.
“They are now being asked to obtain passports at very short notice, creating uncertainty for people with existing travel plans.”
She warned that businesses in her constituency and other areas with close ties to Ireland could be affected.
One such business, Teithiau Elfyn Thomas, a travel company based in Ynys Môn, has raised concerns about potential financial losses due to the sudden change.
Ms Medi said: “Many of their customers are elderly people who do not currently have passports because they have never previously needed one for these journeys.
“Expecting people to secure passports within such a short timeframe is simply unrealistic and risks causing significant stress, disruption and financial hardship.”
The MP has written to the Home Secretary, calling for a longer grace period and additional support for those affected.
She said: “The Government must act urgently to mitigate the financial impact on businesses and travellers.
“That means providing a longer grace period, ensuring passport processing capacity is sufficient to meet demand, and exploring support or flexibility for operators and passengers who stand to lose significant sums as a result of these changes.”
She also said the issue extends beyond her constituency, and she has raised it with other Plaid Cymru MPs due to its wider implications.
Ms Medi said: “Rather than forcing people into a last-minute scramble, the Government should provide a longer grace period and work with ferry operators to ensure passengers have adequate time to prepare.”
The new requirements were announced on August 26.