A summary of items found while googling:
A summary webpage on Brexit and Ireland. This details the agreements that have been made but not ratified - the Withdrawal Agreement, and the "backstop" Protocol. Does not cover "no deal".
Smart Border 2.0: a plan for minimally intrusive border checks, but as far as I know this is not ready for implementation. This is the nearest answer to "plans to avoid a hard border", and is roughly the "high tech border" that David Davis keeps mentioning.
"There will be a need of a Customs and Border solution post-Brexit on 29 March 2019 at 23.00, regardless of political solution and Brexit negotiation results.
It will have
severe consequences if such a Customs and Border solution is not designed,
developed and implemented to facilitate the movement of people and trade."
- The Joint Report: "In the absence of agreed solutions, the United Kingdom will maintain full alignment with those rules of the Internal Market and the Customs Union which, now or in the future, support North-South cooperation, the all island economy and the protection of the 1998 Agreement."
i.e. the UK government has agreed to "Brexit in name only" until this is resolved, and not immediately start importing US hormone beef etc. However this is only an agreement by the executive; normally that wouldn't be significant but at the moment it matters that Parliament has not agreed to it.
The Commission says there will "obviously" be a hard border but provides no detail, while in the same article the Irish government denies having plans for this.
A paper on the various electricity and gas markets. While these are likely to continue operating even if the legal basis becomes unclear (q.v. the gas pipeline happily operating across the warzone in Ukraine), that could break down in the event of a dispute, and it makes it impossible to build improvements without clarification.
Further sources (see "related")