An Post has asked customer not to post mail for addresses in the Drogheda, Co Louth or Tuam, Co Galway postal areas due to industrial action by members of the Communications Workers Union.
In Galway, outbound mail to be transferred to Athlone, Co Westmeath for automated processing is causing a delay of up to two days in mail leaving the area.
It is also causing delays in incoming mail.
While delays in deliveries of more than a week for regular mail and up to two weeks for larger envelopes and packets are being experienced in Tuam, Co Galway.
Ongoing unofficial industrial action in Drogheda, Co Louth over the method of paying overtime has forced the company to seal mail boxes and to appeal to customers not to post in the area.
Mail deliveries are running up to two days behind schedule.
An Post has also informed its customers that the industrial action will result in delays to all mail posted in or to Galway City and County.