Ireland has historically received a meaningful share of its Asia-origin cargo via the UK landbridge — sailing to a UK port, trucking across Great Britain, then a short ferry crossing to Dublin or another Irish port — because it was often faster than a direct deep-sea call. Brexit changed the economics of that route by inserting a customs border between Great Britain and the EU (which Ireland remains part of), adding a clearance step to what used to be a seamless road transit.
Direct deep-sea services into Dublin or Cork have grown in relevance as a result, avoiding the GB customs step entirely at the cost of a longer transit time than the landbridge previously offered, since direct Asia–Ireland sailing frequency is lower than the dense UK port rotations most Asia–Europe services call.
Which routing makes sense depends on the shipment's time sensitivity and value — for cargo where the extra week or so of direct transit is acceptable, avoiding the landbridge's GB customs formalities simplifies the process; for genuinely time-sensitive cargo, the landbridge can still be faster despite the extra customs step, if the formalities are handled efficiently.
Irish import VAT and customs procedure otherwise follow standard EU rules at the point of entry, regardless of which routing is used to get there.