Why are some species not found in Ireland?

[Dave McCormick]

I am writing a report (PDF) on butterflies and moths in Northern Ireland, more so on rarer species giving records on them and when they were last found and those that may have been seen, but were unconfirmed. Reading the book "Butterflies and Moths of Northern Ireland" and it asked a question in it, why are so many moths and some butterflies found in Britian, but absent from Ireland? Some like Adonis blue have an obvious reason, lack of foodplant, in this case Horseshoe Vetch.

But for some species like the Glanville fritillary, it could exist in Ireland as its foodplant Ribwart plantian does exist the same can be said for Broad-Bordered Bee Hawkmoth, it feeds on Honeysuckle and it can be found in many places all over Ireland. But the other question was why Real's wood white has not been found in Britian either as well as the Irish Annulet moth.

I am guessing for some moths and possibly some butterflies too, they migrate from the continant, and find enough of what they need in England and don't migrate any further and don't make it to Ireland. Some could also be absent in Ireland for climate reasons as parts of England get warmer than it does in Ireland. Also the species found in South of England are usually in a warmer climate than those further north and it could be a reason why some are not present, but for others and some like I stated above, not sure why they were never found in Ireland