The RoI can block / veto any Brexit trade deal, if they are not satisfied on the border issue.
Barnier has said talks will not start on trade until sufficient progress has been made on the 3 main "housekeeping" issues: NI, expats, exit bill
I doubt if he would consider that telling the RoI they should Irexit - or else - is sufficient progress
(Spain could also veto if dissatisfied with situation for Gibraltar.
Germany, France & others might veto if angered at DD's attempts to divide the E27 and stir up trouble ...
the EU parliament could block the trade deal)
The problem about chaos & delay on the UK side is that the Uk economy will suffer far more proportionately from a no-deal Brexit than the E27
An assessment of the economic impact of Brexit on the E27
Very detailed study for the EU parliament with many refs, graphs & charts in appendices.
I wish the Westminster Parliament had a study of similar depth
I know, "fed up with experts and their inconvenient facts"
http://www.europarl.europa.eu/RegData/etudes/STUD/2017/595374/IPOLSTU(2017)595374EN.pdf#page388_
Abstract
"For the E27 the losses are found to be virtually insignificant and hardly noticed
in the aggregate.
By contrast, for the UK, the losses could be highly significant, over ten times greater as a share of GDP.
Impacts on various member states - in particular Ireland - and sectors in the E27 could be more pronounced
< I would expect they can be supported by other EU countries not economically much affected >
Conclusions
Our main finding is that available studies largely agree that Brexit will inflict losses on both sides.
All studies agree that the losses will be considerably larger for the Uk than for the E27.
Only in very pessimistic scenarios would the losses for the E27 reach a significant size
....
The available evidence suggests that the additional losses that would result from a bad or uncooperative outcome would be born mostly by the UK
In the US - if Leavers don't want to read EU articles - the Brookings Institute commissioned a study
"Brexit's long-run effects on the UK economy"
which did not see sunny uplands:
https://www.brookings.edu/wp-content/uploads/2017/02/brexits-long-run-effects-john-van-reenen.pdff_