Following an election and when all seats are filled, I suggest that unelected  candidates are placed on a panel in order of the result. 

If one of the elected TDs, Seantors, MEPs in the constituency resigns / dies or vacates, the seat will be offered to the next placed candidate. 

Similar to co-opting – but the panel is a reflection of the electorate.

Rules can be placed around timelines etc e.g. time after election, time to accept.

Benefit – saves on byelections, time & cost. Loss of representation is minimised.

Thanks Maurice