The number of elected members for each local authority area is set out in Schedule 7 of the Local Government Act 2001, as amended by the Local Government Reform Act 2014. The reviews of local electoral areas now underway are based on there being no change in the total number of elected members in any of the thirty-one local authorities.
In the case of Cork specifically, I understand that the change in the population of the city and county that will arise from the proposed boundary alteration will result in the ratio of population to councillors being broadly similar in both local authorities, whereas currently the ratio in the county is much higher than in the city.