Ireland can now certainly support Great Spots, and probably Greens as well, imo. You should see some of the crappy arable habitat that they breed in in England, commuting between small patches of trees to make up a decent territory. I know one pair breeding in a copse of 13 trees, and flying 400m to another copse of 12 trees to feed. If enough of them made it to Ireland to create a self-sustaining population, then they'd be away. But a single pair isn't enough to create a population. All it needs is a sparrowhawk to come along, or a heavy rain storm flooding the nest, or any other stochastic event, and they're gone. But the British race is probably genetically prone to not undertake water crossings (or else they'd fall off then island, so it's not in any individuals interest to have the urge, so it's probably been selected against), so not many of them are ever going to reach Ireland. Even if one or two pairs are breeding now, that's not really a colonisation, as England has had 1-2 pairs of breeding bee-eaters, common rosefinches, hoopoes etc in some years and nobody considers that a colonisation - it's an extra-limital breeding event.