Thanks Tavern - so it's possible that one is a phonetic variation of the other, that the difference between the two is due to a scribe's interpretation of a surname he's heard rather than seen. There's not a lot we can do with the thing at this moment, though - all we can do is file it away and keep an eye out for clarification from other sources.
There's something else I'd like people to consider - one of the names at the bottom of James Halpin's will is "David Keely". When I first read that name I began racking my brains trying to remember where I'd seen it before. A day later it occurred to me out of the blue - cast your mind back to Reply 246 (Tues. 13 July 2010) on the previous thread. There you will find an extract from Thomas Mathew Halpin's Disclosure Connected With The Late State Prosecutions In Ireland..., in which he cites testimony from his father-in-law, James Keely. Not the David Keely named in James Halpin's will, but a Keely nonetheless. Idiosyncratically spelled surnames that coincide in this context suggest to me that chance is not at work here, but something more significant.
I was often told that over the years the Halpins parted ways on account of strong political differences. There were many opportunities for differences between family members to arise - the terrible events in 1798, for instance, in which 30,000 died and a James and William Halpin, "distillers", were described by a Castle spy as being particularly virulent United Irishmen and prominent in the unsuccessful attempt to save Robert Emmet from the gallows in 1803; the Young Ireland Rising in 1848 (in which T M Halpin features above); the Fenian Rising in 1867, featuring Gen. William G Halpin, and of course the 1916 Rising, involving my own immediate family. So, given an albeit tenuous link between surnames, it's not out of the question that the Halpin and Keely mentioned in James' will, were related to the Halpin and Keely mentioned in the "Disclosure..." - these links may suggest strong sympathies for the political vision of the Society of United Irishmen. I've touched on this question before and I'll ask it again here - what was James Halpin, father of Captain R C Halpin, doing with himself around the time of the execution of Robert Emmet?
Something else to keep in mind, I suppose.
Finally - does anyone know anything about James Halpin's son Thomas? You'll note the continuing repetition of names, I hope.