" I am desperate to get hold of this song! "
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Well, Feargal, how well-acquainted are you with music on computers and with using MIDI? Give us a little information about what you need.

If you have music software, you can go to this site:

http://www.contemplator.com/carolan/complt.html

and search for 'Irwin'. Left click on the title to hear the tune, right click on the title to save it. Let's hope that people here are correct in saying that the song you want is based on 'Planxty Irwin.'

The beauty of MIDI software is that it gives you power over the song. You can change the key, change the speed, alter notes, change elefs, print it, write new parts, etc etc.
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But perhaps you use ABC. I bet you could find an ABC file of it at JC's Tunefinder, which is a wonderful site with traditional tunes in many formats. Come to think of it, go there first and see what you find.