People from Spain are not particularly dark-skinned, unless they’re from Southern Spain, where there is more of a North African influence.
The Irish would have been descended from a group located in Northern Spain, where many people have very black hair, very fair skin, and either blue or black eyes. This is basically the look that the “black Irish” have.
Things like hazel eyes and even red hair are generally the result of a mixture of a blond group with a black-haired group, such as the mixture of the blond Vikings with the original black-haired Irish.
There are virtually no red-heads in Northern Spain unless they are the product of modern marriages.
In any case, Latin Americans are an entirely different mixture. They are European Spaniards with a Native America admixture, combined with later influences of other European groups, particularly Germans and to a lesser extent, the English and the Italians. Some are more or less Indian than others. Argentinians, for example, have very little Indian blood and pride themselves on being mostly Italian (probably why their country looks so good but is so dysfunctional).
As for the Spaniards, a lot of the ordinary people who did much of the settlement came from Andalusia (Southern Spain), although most of the expedition leaders and officers came from Northern Spain, so there is a wide range of coloring and features.
>> Argentinians, for example, have very little Indian blood and pride themselves on being mostly Italian <<
It’s often said of the Argentinos that they are Italians who speak Spanish — and who pretend they are English!
(They take afternoon tea at the polo club, etc.)