invasive or not? do i let it go or kill it?
No interactions present.
Even if it were a Harliquin, there would be little point in killing it, they are so well established it would never make any difference.
And remember we still don't know how much, if any, damage the Harliquin may be doing to our native species. Many non-native species don't cause any damage at all - but others of course do tremendous damage.
And of course, is Harliquin even an alien speices? It could be argued it flew here naturally from Europe and got established that way. I mean we don't go round shooting every migrant bird which happens to reach our shores...
The definition of an alien species, as defined by the convention on biological diversity, includes natural spread from an introduced population, so the Harlequin is officially an alien species in Britain, despite never officially being released here.
We're increasingly confident that the Harlequin is having detrimental effects on native ladybird species (eg this paper: http://onlinelibrary.wiley.com/doi/10.1111/j.1472-4642.2012.00883.x/abst...), but I do agree with your point about not killing them - there's just so many that there's no point, plus they can be relatively tricky to ID so natives would end up being killed instead.
Record your ladybird sightings!
with the map location.
in Dublin. Or does this species not occur in Ireland?
Lat/Lng: 53.3498, -6.2603
OS grid ref: SG165921