J&J Vaccine - Booster Required after Just 3 Months? Panic Decision?

So, my question is - why are guidelines being drawn up based on such selective data?

I would say governmental panic because of a clear uptick in cases and the threat of overwhelmed health services, even in the face of a high percentage of vaccinated in the population is a large part of it.

Because though slightly lower overall than a full course of the two dose, the single dose of J&J showed a much higher effect than a single dose of the two dose vaccines, hence one was originally deemed sufficient. Indeed the two dose are now looking at having a third booster. A second shot of J&J at six months is as strong or stronger than three of the mRNA. Having a "one and done" approach was more than a "niche" too. For a start it doesn't require specialised refrigeration unlike the mRNA vaccines. It can be stored in a domestic fridge and lasts longer, so is a much better bet for use in less built up and advanced parts of the world and required only the one shot to give decent protection so much less of a requirement for a system of follow ups and that protection is in play more quickly. It's also cheaper than the mRNA vaccines, even though that tech has been promoted as the quicker to market cheaper option, and that was cheaper per dose with it. A requirement for two doses over doubling the price again.

if the reports around waning efficacy are to be believed (and I have seen nothing to convince me otherwise)

Then maybe read the opening post on this very thread and look further? It's literally stated there:

When compared to a South African study across 480,000 individual healthcare workers which was reported in this article: Johnson and Johnson vaccine effective against Delta variant (irishtimes.com) and goes on to mention: It’s probably more protective against delta than it was with the earlier beta strain, she said in a presentation Friday. The single-dose shot was 71 per cent effective against hospitalisation and as much as 96 per cent effective against death, she said. It also demonstrated durability of eight months. “These results show there is no need for a booster yet,” said Gray, who is president of the South African Medical Research Council.

Far more numerous studies conducted across hundreds of thousands of test subjects in a few different nations and including the Delta variant have shown that efficacy remains high over a studied eight month period with the Jannsen vaccine and even today with talk of boosters all the rage researchers both inside and outside of J&J repeat that, but now add that a booster ideally at six months (or longer) gives even more protection, as far as antibody levels are concerned. Going on that the notion of "250,000 additional unvaccinated people to deal with" is more than a little melodramatic. Going by your assumption we're also dealing with the rest of the vaccinated Irish and their waning Pfizer/Moderna/AZ vaccines in need of boosters as more "additional unvaccinated" numbering in the millions. Eh, no...

Durability means what the word means. The positive effects of the vaccine on serious illness and death from covid 19 remained durable over time. It was in human trials since the middle of last year and those subjects and others since in wider studies have been followed up, including nearly half a million healthworkers in South Africa where Delta's the main strain. All of this info is in plain English in the first post and the links contained within it. I'm not seeing the confusion.

Yet your first post out of the gate suggests you're convinced about waning efficacy, it seems based on a single study, or more likely a media report on same. You then take a leap that would shame an olympic long jumper that this means we'll have quarter of a million unvaccinated people in Ireland. Then rewrite history regarding boosters where apparently they were "fully aware" that these would be required. They weren't. There was talk about maybes and how long immunity would last, even what type of immunity(active antibodies/Memory cells) would have real world impacts, but vanishingly few were talking about boosters until the Israeli experience and mostly Pfizer and mostly in the elderly demographic.

Well unless someone independently brings in the J&J or another viral vector, I for one will not be getting a booster based on mRNA.