Per an earlier post, roses and apples are in fact quite closely related. Anyway all the Gaelic languages are scions of the same root language and while Welsh does seem to be unique when written, it's quite likely any profound present-day speaking differences crept in due to revivals- combined with regional accents- as much as anything. Remember Patrick, he could travel the Celtic world and immediately be understood even though his speech was considered 'strange' by some.