Here's a better explanation.http://www.environ.ie/en/LocalGovernmen ... 895,en.pdf
An elected candidate's surplus is distributed based on the next available preferences for continuing candidates (i.e. candidates not elected or excluded) contained in the last parcel of votes that brought the elected candidate over the quota.
In the case of a surplus arising on the first count, this parcel is made up of all the elected candidate's
first preferences. In any other case, only the last parcel of votes received by the elected candidate is involved.
Where all the next preferences in the last parcel of votes received by the elected candidate cannot be distributed because they exceed the surplus, the surplus votes are distributed to candidates still in the running in proportion to each one’s share of next preferences in that parcel of votes. The votes for inclusion in the surplus are taken from the top of each candidate's sub-parcel of next preferences made up from the last parcel of votes received by the elected candidate.
So it seems that the number of votes to be transfered is calculated based on the percentages of the most recent parcel of votes that got the candidate over the quota (in the first count this will be all votes, in later counts the most recent parcel of transfered votes).
Then the actual physical ballot papers that are transfered to each candidate are picked at random, by taking them from the top of the sub-parcel of votes that transfered to that candidate.
Unless I am misreading the explanation, which seems somewhat ambiguous.