I’ll concede that’s an argument that isn’t totally without merit, however it fails to take into account the huge imbalances in power between the UK and the ROI (as well as England and Scotland, although that’s a separate argument).
I expect a united Ireland would economically benefit England, long term; however the UK leaving the EU would almost certainly significantly damage the ROI, something which is never mentioned or even discussed by proponents of Brexit as an unfortunate side-effect of the process.
Therefore, an act of putative national self-interest that renders our closest friends and neighbours to be worse off, given their reliance upon our engagement with Europe, is distinguishable from an act that will most likely be of benefit to England, economically at least - and is therefore something to be ashamed of.
Selfishness only really counts when your selfish act impacts negatively upon others, especially if you’re all too aware of any such likely impact.