from Edward Bunting, The Ancient Music of Ireland (Dublin 1840), Page 27: Double notes, chords, etc. for the right hand.
Irish malairt phoinc
spoken by Gráinne Yeats
Scottish Gaelic malairt phong spoken by Tony Dilworth
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“Or crossing the hands, the right hand taking the place of the left.”
Simon Chadwick 2008
This very enigmatic figure is plain enough in the notation - the left (treble) hand plays a repeating three note pattern, which the right (bass) hand accompanies first below, and then above. The footnote does not really describe what is happening; the right hand is not ‘taking the place of the left’ but instead is reaching up to play its bass figure high in the range.
The musical example given is a fragment from Bunting's piano setting of Saley Kelly by Carolan. In both the 1840 print version of this tune, and ms33(5), this figure is labelled ‘Malairt phonch’. However none of the other ms settings include such a figure; the ms33(3) version is the only other one with bass indication, and here the bass line stays resolutely below the treble. The ms29 draft has no bass indications at all, which is very suggestive that the harper Bunting collected that version at least from, did not use such a distinctive bass figure. Of course it is quite possible that the tune was independently collected from multiple sources.
See Alasdair Codona's discussion of this term, and his comment on the grammar.
Simon Chadwick 2008
Malairt phonch - To reverse the hand
Probably for malairt phoinc, the literal meaning may be 'change of point', taking the second element to be the genitive singular of ponc, 'a point' in a range of possible applications (including 'a dot', 'a detail', 'a moment'). Malairt, in any case, means 'a change, exchange', but the second element might also be phonc, genitive plural, thus 'a change (or exchange) of points'. On p.28, fourth column, the spelling is Malart phonck, and the term is explained as denoting one of the two 'parts' of port.
On p.34 Malairt alone is explained as 'Change of the hand'; and on p.97 Bunting tells us about the performance of the tune Saely Kelly (printed on p.27 of the main text): "Part of it is played by crossing the hands, technically called in Irish, Malart Phonche, and shews the degree of perfection to which the older harpers carried their performances".
Colm Ó Baoill 2002