Hi,I agree with the tinwhistlers mutha. Learn a few of those tunes listed. They are always popular and the rest will join in with you, often doing elaborate harmonies which make even the simplest of tunes sound great. (Planxty Irwin being a good example of this).
I also recommend Star of the County Down because so many other songs and tunes are very similar and you'll soon pick them up.
Slainte
Alison