Traditional Irish music, as it can be known in session like an instrumental form, is a dance music. That is to say that we will play melodies, whose rhythmic form is the basis for the dance. These melodies have a very particular structure, so it is important to know this vocabulary.
Most of the pieces work in the same way, they have a part A and part B of 8 measures each and every part is repeated twice :
there are always exceptions: tunes with 3/4/5 parts, tunes with 8 or 4 bars etc ... So many traps which requires to know the melody before harmonization.
Playing tune just once would last only 30 seconds. So we usually build a set with 3 pieces of the same dance that we will play 3 times each (other combinations are of course possible). Following that, the set of tunes will last about 4 minutes. From a structural point of view it would give this example for a 3 piece suite played 3 times
Tunes 1 (AA'BB' - AA'BB' - AA'BB')
Tunes 2 (AA'BB' - AA'BB' - AA'BB')
Tunes 3 (AA'BB' - AA'BB' - AA'BB')