It would not matter even if the majority was less than 11. The way to think about this is to ask the question; Would a normal Seanad vote be valid if 11 members were absent? The answer is Yes.The Seanad provisions are a bit more definite: "There *shall* be sixty members". Would the votes/motions of a Seanad with 11 out of 60 members still unappointed actually be valid? Again, I suppose if a Seanad vote/motion was carried by a majority of more than 11 votes, then in a certain sense the 11 unnominated Senators would not matter...
So the quorom/minimum attendance thing is a bit of a red herring. Its a vote taken among whichever members show up for the vote.
Something of more significance would be the spectacle of a newly appointed Seanad voting on some piece of legislation along with the previous Dail. Regardless of which house proposed the legislation, its still an odd situation. But again the constitution does not specifically forbid it.
The new Seanad is required to be formed within 90 days of Dail elections, therefore it could be argued that there is "an implied" 90 day deadline for the formation of a new government. We can certainly say that it was envisaged that any govt. would be formed within this time, hence the reason for the deadline (so that legislation would not be held back by some previous Seanad still hanging around)
It is obvious the FG and FF leaders are conspiring together to drag out the period of this caretaker govt, knowing that (due to the numbers) no new govt. can form without the permission of at least one of them. That is a very poor political behaviour. Only borderline unconstututional, but they should have the integrity to avoid pushing the rules to the brink.