Nearly every house in Scotland has a Harmonica, more often called a Moothie, in a drawer somewhere & it's so easy to play one that most youngsters, years ago, would have had a go at it.
Listen below, to a Scottish Moothie player, the great Donald Black, & check out his website while your at it:
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Scottish Pipe Jig: 
Danns An T-Sabhail: 
Shetland Reel: 
Nigel Gatherer tells us that - "The moothie (or mouth organ or harmonica) has been played in Scotland since its popularisation. Mouth organ bands were once common and it's still a fairly widespread instrument to be found today."
However that popularisation that Nigel talks about only really took place from the Vicrtorian period. You might like to check out Nigel's 'Moothie Players' page to get an idea of just how popular this instrument is in Scotland: 
It's not an instrument that is played out very often here in Ulster, but it is known by many as the poor mans Accordion & sounds just great when played alongside a Fiddle. It is often affectionately referred to as the 'Tin Sandwich' & I personally think more use should be made of it.