Is there any possibility that the British might have started something? (Intentionally or accidentally).
For example if when the 1970 Arms Crisis breaks the British Government manage to misinterpret it as the Irish military intentionally supporting paramilitary groups north of the border?
Or perhaps a British patrol accidentally crossing the border and having some sort of encounter with the Garda where they refuse to let themselves end up in Irish custody?
It doesn’t really need the British to use force to punish Ireland for any actions that London disagreed with, this is when Ireland is overwhelmingly economically dependent on the U.K. market, a simple tariff on Irish goods and soon enough Dublin would have to change positions if that was the issue.
Given how much intelligence services would have known about Irish government and Army plans/operations I have little doubt that they knew exactly what was going on in the Arms Crisis as it happened, so I think you’d need something more than that. However the options are limited, as has been pointed out the Army capabilities are highly limited at this point, hell our Ranger Wing is only forming at this point so even “unconventional” support isn’t an option.
As to border incidents, it happened regularly enough, depending on the unit, some respectfully got back onto their own side, some didn’t. But when you consider again nothing happened even up to the DUP “riot” at a Garda station, it’s unlikely to lead to any military action, at most some snarky diplomatic language.