I have seen tunes defined as Irish Airs. What factors determine this description? Is it simply a melody played at a slow tempo? Is the air played in 2/4, 3/4, 4/4 timing or something different?

Also what determines whether a tune is a "waltz"? Is every melody composed in 3/4 time a waltz or is there some other criterion?

Thankss for your help with this.