US/Israel conduct airstrikes on Iran again

OK, a couple of things to unpack here.

Firstly, the use of the Shannon stopover is in accordance with the long-standing Irish interpretation of the obligations of neutral powers dating back to The Emergency. The Irish government always distinguished between combat operations and administrative moves of the various combatants which came into its jurisdiction (normally air crash or shipwreck). The former were returned to their side of origin, the latter interned. The interpretation ultimately may be wrong or right, but at least Ireland has been consistent for the last 80 years, and an argument it's wrong is going against the weight of precedent.

Complicating the issue is that the treaties on the obligations of neutral powers pre-date air transport and can only be used as a guide, but Art 7 of Hague XIII (The Rights and Duties of Neutral Powers in Naval War) states "A neutral Power is not bound to prevent the export or transit, for the use of either belligerent, of arms, ammunition, or, in general, of anything which could be of use to an army or fleet." Switzerland actually does prevent such (much to the dismay of the Swiss arms industry of late), but that's a domestic policy, not a legal requirement of a neutral power and why the position was not well received by other European countries trying to arm Ukraine. It wasn't "You're neutral and we respect the choice not to allow arms to Ukraine", it was "OK, you're neutral, but that doesn't mean you have to prohibit it, why are you being dicks about this?"

With regards to Oman and its support to US Navy vessels prior to hostilities, it's worth looking at Article 13 which specifically covers what happens when a belligerent vessel is in port when the fighting starts: They are to be instructed to leave within 24 hours, but the fact that they were present in the country taking advantage of the port does not strip the country of its neutral status or protections.

So here's the thing about neutrality: There is no "register of neutral countries." A nation can have a declared default position of neutrality, but neutrality itself is always taken on a case by case basis with respect to any particular conflict once that conflict starts. Simple historical example: During WW2, Japan and Germany were at war with the US and the UK. The US, UK and USSR were at war with Germany. However, the USSR had a policy of neutrality towards the Japan/US fight, and Japan had a policy of neutrality towards the USSR/Germany fight. As a result, US combat equipment being sent to the USSR by lend-lease was permitted by Japan to sail right through the Pacific war zone to be used against Germany. If Oman declares "You two have just started fighting, leave us out of it", then that's legitimate neutrality.

The important part of neutrality is even-handedness, Article 9: "A neutral Power must apply impartially to the two belligerents the conditions, restrictions, or prohibitions made by it in regard to the admission into its ports, roadsteads, or territorial waters, of belligerent war-ships or of their prizes." As long as Oman makes available the exact same facilities under the exact same conditions to the Iranians as they have to the Americans, they are still considered a neutral power. The Iranians may choose not to ask the use of the facilities of Oman, but as long as they are treated equally, it's legit. Or at least would have been before Iran started attacking it, in which case there's an argument for it being justified.

As another example of the relative archaicness of the laws, a belligerent ship may use the port of a neutral power to refuel, either to the limits of its bunkers, or with enough to get back to its home nation (Art 19). In practice today, this means that the ship can be refuelled for the duration of a local war. It may not, however, use the neutral port for this twice within three months. However, there is no indication that Oman has not adhered to the requirements of the Convention. Which it never actually signed. Actually, Ireland's not a signatory either, but anyway…