Found it! It's "Roche's Favourite" which I found after search for "set dance James Galway Chieftains" rather than just "Chieftains." Feel a bit silly for posting now, but I'm pasting a link here in case anyone is interested... really lovely tune.
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Mary welcome to the session - i just had a quick listen to Kevin Burke and Michael O Dhomnaill on You tube and it sounded like the chords were Dm Dm F G Dm Dm Dm Am ||
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