Not much actual writing since Friday, but a good bit of looking at maps and reference books and thinking. Conclusions: we know very little about Britain in this period (other than what Tacitus has to say); we know even less about British Druids (i.e. they may have existed, and if so were probably on Anglesey at some point in the 1st century AD, but we have no idea what they were doing there); and we know rather less about Ireland and Irish Druids (if any). Ireland was where it is today; there was some sort of (probably complex) "Celtic" society there, speaking (probably) a Godelic language; there were probably religious / magical specialists of some kind (who may or may not have called themselves Druids) in that society at the time in question; there is a possibility (supported to some extent by archaeological discoveries of Roman material) that Agricola may have sent some sort of military expedition to Ireland, most probably in late 81 AD, with outcome unknown; if he did send such an expedition, they most probably crossed from the vicinity of Galloway (Scotland), as he was operating in that area at that time with the required manpower and naval support; Agricola (according to Tacitus) had in his entourage at that time an exiled Irish noble of some sort who could have served as a focal point for such an expedition. What any Druids (if they existed and were interested) might have had to do this is entirely conjecture.
It's a good thing I'm writing fiction.