The species is an old friend. Because it is a problematic species from a management perspective in the US, it is hunted down and controlled (cut, poisoned and burned!) when it interferes with conservation plans for a preserve. And rightly so, I would say. However, it is attractive, hardy, and a great success in the US - my relationship with it has always been that of pupil and master, or perhaps merely envy on my part. For whatever reason, I have spent the better part of a decade interested in this shrub, though truthfully we still don't know enough about it.
Those of you who know this plant well take a look a see if you agree with me that it is, in fact, buckthorn. If it is not, I will be back in the field in the coming days. The finely toothed leaves are almost opposite. Though it is not fruiting yet the fruits are developing in clusters. Also the bark is much grayer than I am used to seeing in the Midwest.
UPDATE 22nd July 2012: - There were enough differences between this specimen and R cathartica I am used to seeing in the midwest that I spend a little time working on it with some keys later that day. With the assistance of old pals Helena Twomey and Bill Quirke (of Conservation Services) it was a much better fit for spindle (Euonymus europaeus). Fortunately I found buckthorn a few days later in Co Mayo.