Genetically, the irish are nearly indistinguishable from the english, germans or dutch. Spainsh and Italians have more mixed blood from different invasions- but still generally fall into similar haplogroups with the other euro groups. South America does have spanish blood, but significantly differs on average from those euro groups.
Irish, English, Scottish, Dutch and German are still distinguishable from one another. Irish and Scottish are more or less related since they are of mixed Celtic/Germanic (mostly Viking and Norman) stock. The English are Germanic( Anglo-Saxon for the most part with some Viking and Norman admixture). Germans and Dutch are related and very distinguishable from the Brits and Irish. They both descend from local Germanic tribes. No or very little Viking or Norman admixture.
Experts are even able to divide them just by looking at them. But I guess it takes some experience to be able to do that
