What they could have done was introduce a non-urban default of 60 km/h with the requirement of a bylaw to change/raise the limit to 80 or 100 km/h.
No, please, no!
That would result in:
(a) a lot of unsightly signage for all the exceptions
(b) a lot of money paid to already overpaid bureaucrats and lawyers to draw up 'essential legal documents' relating to the bylaws you mention, when the same basic safety requirement can be met by having a one-size-fits-all rule with an emphasis on behaving sensibly (i.e. NSL)
(c) a lot of stretches of roads with absurdly low limits -- especially in Ireland, a lot of non-National roads vary considerably from kilometre to kilometre in alignment, width and surface quality, with some stretches that are clearly safe well above 60 km/h
(d) a general degradation and bringing of the law into disrepute as virtually everyone flouts the limits where they are clearly daft, with no additional resources for police enforcement.
Every time the question of micromanagement of all speed limits comes up, whether on here or in the general media, and whether in the UK or Ireland, no-one manages to satisfactorily demonstrate that a remotely significant number of accidents on rural roads have anything to do with the speed limit. In other words, (a) how many are due to people driving at more than whatever the default limit is, (b) how many are caused by speeds between the current limit and the proposed new range of limits, and (c) to what extent reducing the limit without enforcing it will actually affect people's behaviour. Unless there's a clear answer to these (and other) questions, there's really no justification for replacing a system based on common sense with a much more complicated and onerous system of micromanagement.