Spain is both the easiest and the hardest - Franco was basically pro-Axis, but he was smart enough to realize his country simply didn't have the ability to go to war. If the war seemed already won - if the RAF was destroyed over France, and the BEF was destroyed on the beaches, so that Britain can offer no short-term threat - then that's the start of convincing him to join in. It's tough, though.
Ireland requires the Axis to have already won. There's no possible way, ASB or otherwise, that Ireland joins the Axis without Britain having already fallen, because there's no way that Ireland can stop the British from invading and occupying the country. There's also no significant faction that would want to join the Axis, so you'd need a join-or-die ultimatum, but that's easily possible if Britain is down.
Turkey is a lot like Spain, not really able to fight, but unlike Spain they weren't pro-Axis. If Russia falls and the Axis gives a join-or-die they'd have to at least consider it.