Almost unbelievably, the wallabies were reportedly introduced to the island by Dublin Zoo as a way to humanely handle an unexpected population boom... in the 1980s!
I had to look around for confirmation of that- and I only found a few vague references to it. I certainly hope they are all wrong. Accidental introduction is a problem, but intentional colonization as late as the 1980s by an exotic species seems extremely irresponsible to the animals being introduced and the native animals hosting this new species.
Island ecosystems are as fragile as they come. I find it difficult to believe that zoo officials- presumably animal and ecosystem preservers- would do something with such potential for disaster as recently as this reported wallaby dump.
In any case, environmental soapbox speeches aside, the wallabies are there on the island and they seem to be sustaining a small breeding population. It seems that these shy marsupials are difficult to capture on film or photograph. The only pictures I could find are from the air or from a great distance.
The island is mostly privately-owned, and people can only visit via a special cruise package. Either way, let's hope this cute little invasion stays on the island- I'm sure mainland people don't want these guys hopping around their back gardens!