1st, the ethnicity results are not certain or guaranteed. It is an evolving science. If you peruse past postings on this board you will find multiple instances of people discussing the validity or non-validity of their ethnicity percentages. I was 90% Scandinavian/7% Southern Europe/3% unknown in the original version and my mother was 85% British Isles/10% Scandinavian/5 % unknown. How can that be you ask? (No, it is not an adoption thing, I am her child). The revamped ethnicity has me at 40% Great Britain/35% Scandinavia/10% Iberian/7% East Europe/4% Ireland/3% Europe West/1% Italy-Greece. Is it a "better" prediction? I am not sure. But it makes more sense than the 90% Scandinavian. (My mother went to 37% Scandinavia/26% Great Britain/20% Ireland/10% Iberian/5% Italy-Greece/1% West Europe/1% East Europe.)

Ethnicity based on the spread of your ancestors on paper will not necessarily match the DNA given the random inheritance and recombination of DNA. Some branches will get over emphasized while other ancestors will end up contributing no DNA at all to you. Specifically, on your Irish section look at the expanded information they give you. That DNA is found primarily in Ireland, Wales and Scotland, but also in England and France -- and they state that pretty close to the top of that page.

2nd, as for the range, that is only a prediction. It is based on the averages of DNA inheritance and recombination. On average you will only share 50% of the DNA with a full sibling. But it could be more or it could be less based on exactly what chunks you got from each parent.

Here is a text chart to help show the percentages:
Expected percentage of shared DNA between:
Parent/child/siblings = 50%
Grandparent/grandchild/aunt/uncle/nephew/niece/half-siblings = 25%
1st Cousins/great-grandparent/great aunt or uncle/grandnephew or niece = 12.5%
1st Cousins once removed = 6.25%
2nd Cousins = 3.125%
2nd Cousins once removed = 1.563%
3rd Cousins = .781%
4th Cousins = .195%
5th Cousins = .049%
6th Cousins = .012%
7th Cousins = .003%
8th Cousins = .001%

These are AVERAGES -- at each level out the level can range wildly. I have a 6th cousin who shows up in the 3rd cousin category according to AncestryDNA. That is because we each happened to get the basic same chunk of chromosomes as the generations passed on. Looks like your 2nd cousin and the generation before happened to get the chunks of DNA that your line did not get. If you really want to delve more into this you need to transport your raw data (and theirs) into FTDNA or GedMatch and use a chromosome browser. It is, unfortunately, not something that Ancestry provides to us as a tool for analysis.

--Elizabeth