On further reflection I think the apostrophe after the s only applies to words which take 's' in the plural. For example I think it was always the men's room but the boys' room (where there was more than one boy).

With regard to the apostrophe after the names ending in s, in Dublin we always used apostrophe s even after an s so we get the example of "Hughes's" that I gave above. However we were always corrected for this and told the correct form was "Hughes'". I suppose these things change over time.