If you appreciate a performance of the musical and vocal skill of a group of traditional musicians, you will most likely 'put your hands together' in applauding them. Some advice: Applause

Applause is a way of registering approbation or approval. At informal sessions of traditional music it may take many forms - from exhibitionistic whoops and gulders to a minimalist, barely-uttered 'good man yourself'. Normal hand-clapping may be appropriate, though it is rarely as prolonged as it would be at, say, a recital of classical music. The punter, after all, is not applauding his own good taste.

The inexperienced punter may be somewhat disconcerted by the custom whereby little whoops and screams are uttered while the music is in progress. These expressions of appreciation may not be as random as they seem. An attentive punter may, for example, make a little yelp at that point where the tune has been played once and is now about to be played again (tunes are played at least twice round), indicating (a) that he knows where a tune ends and (b) that he would like to hear it again, which he will anyway. On the other hand, the yelp may come at a point in the tune which is determined by the punter's perception of a particularly fine melodic variation; to the musician who just played it, it may have been a mistake. Or it may not.

That these usages are sanctioned by antiquity can be seen from the following excerpt from a l9th-century guide to ball-room etiquette:
No person during a Country Dance should hiss, clap, or make any other noise, to interrupt the good order of the company. Snapping the fingers, in Country Dancing and Reels and the sudden howl or yell too frequently practised, ought particularly to be avoided, as partaking too much of the customs of barbarous nations...

It will be readily observed that the practice of these gestures may be hazardous and liable to misinterpretation. The inexperienced punter will be relieved to hear that there is at least one cast-iron, unambiguous way of registering approval: that is, to buy the musicians drink.

The most convenient and correct way of achieving this is not to ask the musicians what they are having, but to get the barman to set them up a round. The barman will then indicate that the round was bought by 'the boy up at the bar' (where 'boy' means a person of either, or indeterminate sex). The musicians may acknowledge the gesture by raising their glasses bows, flutes, or whatever; the punter will respond by raising his glass slightly, giving a barely-noticeable 'thumbs-up' sign, or the like. On no account should the impression be given that the punter has done the musicians a favour, or that they have become, by this mere commercial transaction, indebted to him.

This, by the way, is one of the few instances where one buys a drink for someone without any expectation of being bought one back. The round system in Ireland has many complex conventions which may shift according to geographical or temporal location, but it may be generally observed that if there is no such thing as a free lunch, there is no such thing as a free drink. People unaccustomed to the conventions (Germans, Americans, etc.) sometimes assume that the Irish are an incredibly generous nation who will, if you allow them, buy you drink all night. The reality may be that the Irish like to appear generous; the round system allows homage to be paid to the stereotype, while secretly acknowledging that everyone must stand his turn.

From the Appletree Press title: Irish Traditional Music.
Also from Appletree: Irish Pub Songs.
Also from Appletree: A Little Irish Songbook.