Any county that does not cover, under the federal "Social Security Act", as amended, salaried employees whose duties are directly involved with the provision of law enforcement or fire protection as certified by the county may elect coverage under the statewide defined benefit plan established in part 4 of this article and the statewide death and disability plan established in part 8 of this article by filing a resolution of affiliation with the board. Election of coverage under the plan is irrevocable.