You're 90% of the way there.
The US is also sovereign and if things would become really bad the government would just ban exports.
Ireland wasn't sovereign and it turns out, as much as other countries act brotherly (not that the UK really did), nobody really cares about you like you care.
The UK parliament thought it a good moral lesson Irish should starve because they were the wrong religion
Banning exports would be legitimately catastrophic geopolitically.
Tariffs are mostly just politics and pretty heavily partisan. Banning oil exports would be objectively evil.