Thank you for your post.
We are not sure whether you are experiencing delays when using NCTS in the UK or in Ireland (as you have referred to using fallback procedures at Dublin Airport), and likewise whether the Authorised Consignor approval referred to is one that was issued in the UK or in the EU? The question might therefore be more appropriate to the Customs authorities in Ireland?
Nevertherless, this is the page on gov.uk showing NCTS service availability and issues: https://www.gov.uk/government/publications/new-computerised-transit-system-ncts-email-service-availability-and-issues/new-computerised-transit-system-ncts-email-service-availability-and-issues. Currently it is reporting a full service is available.
As far as when to use fallback procedures are concerned, for a UK AC approval this will be set out in the formal approval letter, but generally for UK approvals it will include something along the lines of:
However, should it be necessary, you may immediately revert to using the 'business continuity procedure' i.e. the paper Single Administrative Document (SAD) when the customs system or the network between traders and customs is unavailable (as a result of a problem with the customs system). When the customs system/network becomes available the details will again be posted on the HMRC Web site. You must immediately return to the input of electronic declarations. Details of the business continuity procedure are given in Appendix 2 to this Annex.
As an authorised consignor you are also approved to revert to the business continuity procedure if your system becomes unavailable or the network between your system and the customs system becomes unavailable due to a problem with your own/local system. You do not need to inform your control office of your decision to revert to the business continuity procedure. However, for assurance control purposes, you must retain details of all business continuity documents used in your records and record the date and time that you started to use the business continuity procedure, and the date and time when your system became available again.