Ireland don't need to agree to allow the EU to impose a border because they agreed to that long before brexit.
Well, quite...
In order to avoid a situation where, by law, they have to have a border, NI needs to be in the CU until there is an acceptable alternative.
And therein lies a problem as Northern Ireland is not part of Ireland's sovereign territory and hence not under its jurisdiction. While the UK has plenty of reasons to co-operate with Ireland when it comes to how Northern Ireland is governed and how it operates, the fact is that Ireland does not have any right in international law to determine how Northern Ireland is governed. So it is a bit of a problem (to say the least) if Ireland's border policy depends on how NI is governed, because Ireland legally has no say in that.
(Ireland) can't agree to any of the UK's proposals regarding the border because every one of them would mean the EU would require us to have a customs border.
But, as you have pointed out yourself, Ireland has already agreed to the requirement for a border (or at least customs checks) with the EU by virtue of signing up to the Single Market.
The UK has and always has had a
right to leave the EU - and as such Ireland should now be acting accordingly. The biggest problem is that both the EU and Ireland seem adamant that part of the UK can never leave the EU because, if it does, Ireland would have to impose customs checks which it has already agreed to with the EU, while at the same time saying that they are totally unacceptable to the UK... you can't have it both ways.
Ireland can still honour both agreements by instating customs checks away from the border. If Ireland (or the EU) impose a hard border, they will be in breach of the Good Friday Agreement. If Ireland refuse a 'soft' border (customs checks away from the border) then they will be in breach of EU law. All other options you mention require the UK to act in a way that is conducive to Ireland's interests, but the UK is not compelled by any law to do so. The only consideration that the UK is bound by is the Good Friday Agreement, and that doesn't say anything about customs checks in Ireland.
Provided that such a deal contains a legal guarantee.
The Withdrawal Agreement itself is a legally-binding international treaty.
They will not accept a deal that might result in a hard border. That doesn’t mean that they won’t impose a hard border if that’s what it comes to.
Sorry, but you're way off the mark with this comment. Ireland will never impose a hard border, nor will the UK. What the Irish government are objecting to, however, is the return of customs checks in any form.
As my discussion with
@Woodybobs illustrates, if there is No Deal, then Ireland are bound by their agreement with the EU to impose customs checks - or they will be kicked out of the Single Market. The EU will likely never do that, hence why the EU and Ireland have come up with a plan to implement customs checks away from the border that would safeguard the integrity of the EU Single Market while avoiding a 'hard border' i.e. physical infrastructure at the Irish border (that caused so much trouble in previous decades).
My bone of contention is, however, that the UK's current proposals are virtually identical to what the EU and Ireland seem to have already planned in the event of No Deal - which is minimal customs checks away from the border. If it is going to be done after No Deal anyway, then why is it so unacceptable?
The last I heard was that they rejected the idea. Where did you get information that they actually agree with it?
Ireland have rejected the idea of signing a deal that creates new customs checks, but Ireland
would have to agree to new customs checks in the event of No Deal otherwise they will be in breach of EU law.
Ireland are gambling that, by refusing to sign up to a deal that creates new customs checks, the UK will essentially give in and either cancel Brexit altogether or agree to leave Northern Ireland permanently inside the EU (and thus effectively out of the UK internal single market) such that no new customs checks will ever be needed in Ireland. The gamble is, however, that if the UK doesn't give in and ends up exiting the EU without a deal, then Ireland will be left with a simple choice between staying in the Single Market and imposing new customs checks, or crashing out of the Single Market by refusing to impose new customs checks. It is almost certain that they will opt for the former.
It’s not, because in the first case they have a choice (to accept or reject a deal resulting in a hard border) and in the second case they have no choice (but to impose a hard border, since there is no deal).
We're not talking about a hard border, but customs checks. My point is that Ireland have already accepted that they will have to impose new customs checks in the event of a No Deal Brexit, and yet are rejecting the UK's proposals because they involve new customs checks.
Timing is also a key issue here - with a deal, no new customs checks would be necessary until at least the end of the transition period, which in all likelihood would be extended until the EU and the UK agree a new trade deal, which could itself eliminate the need for any customs checks - indeed, this has been the UK's position from Day 1.
A No Deal Brexit, on the other hand, means no transition period and customs checks required on both sides of the Irish border immediately. Furthermore, the EU have made it clear that no trade deal will even be negotiated in the event of the UK leaving without a Withdrawal Agreement, meaning those customs checks (that Ireland are so opposed to) would not only be immediate, but permanent.