I'm a volunteer with the Rathdown group, and we're looking at the same question near to you. We were brought together by head office, based on people that signed up to the Get Involved page and are planning an event, leaflet and canvass for the coming months.
The policy is that if there is a group in an area that can self-organise (with support from head office), and if that group can create a structure to support a candidate; at that point (months) there will probably be more formal guidelines in terms of membership/constitution/candidate selection to give a suitably detailed answer.
What I do know is that the candidate selection & branch operations are still being discussed and agreed between members, as is the concept of a SocDem member and what it means for someone to be a member. In the first period up to the election the focus was on the election and now it's moving to a more organisational development discussion/process. Your route to engaging with that process is the 'get involved' link above, it'd be great to have you with us.