The advertising of vacancies is, to a degree, a balancing act. As part of the process, consideration has to be given as to where the best potential candidates are likely to be and how best can I attract their attention and application?
Local paper? Local Job Centre? National Paper? Factory gate? Notice boards? All these should be considered. Certainly internal candidates should have the opportunity and may even be encouraged to apply – career progression/promotion is very important to some employees. Not advertising to those staff is likely to lead to their loss when they move elsewhere to the benefit of some other employer.
At the other extreme only internal advertising and promotion becomes a bit incestuous with little in the way of new ideas or fresh thinking being introduced to the company.
Ideally it would seem sensible to advertise the vacancy as widely as appropriate to its level of importance and seniority. The law only specifically involves itself in N. Ireland under the auspices of the Fair Employment Act 1976 where recruitment processes must be targeted broadly to ensure no discrimination on the grounds of religion, political persuasion or opinion. Great Britain does not have specific legislation to the same detail but an employer does still need to ensure that the wording, placing and processing of adverts is not discriminatory in any way.