You're 40% is not representative of the incidence rate (affected) in the entire gene pool. Four or five dogs is way too small of a sample to represent the entire gene pool.
According to
Wheaten Health Initiative 2009-13 the estimated incidence rate in the UK is 2% (where the 95% confidence interval covers 0.2% to 6.8%) which is not that far off of the estimated 5%-15% in the USA (
AKC Canine Health Fund). I suspect as better diagnostics become adopted we'll find the incidence rates are similar in North American and UK/Ireland. I'll bet you'll find the gene pools of Ireland and the UK are very similar, like the gene pools in MI, IN, IL, and WI for example.
Instead of guessing we could look for the information from informed sources.
The answer you're looking for can be found in a recent copy of the
SCWTCA news letter (page 8). In the USA the breakdown for 880 dogs tested is 33% normal, 48% carrier, and 19% affected; in the UK/Ireland the breakdown for 100 dogs tested is 65% normal, 23% carrier, and 12% affected. I don't know if 100 dogs from the UK/Ireland is sufficiently large enough sample to estimate the rates in the entire population. The rates in the USA match what is predicted for an autosomal recessive trait.
Typically when a new genetic test becomes available the distribution found by the testing is skewed from the predicted for the type of mutation (i.e. autosomal recessive) due to too few samples; once the number of samples is large enough to represent the entire population the data matches the predicted. When the test has been well adopted the new test results no longer follow the predicted due to a sampling bias (testing is performed only for breeding purposes not randomly).
