This thread has thrown up some interesting and well-informed debate on an unjustly maligned branch of folk culture.

My own view tends to coincide very much with El Greko's, and like Leadfingers I am a bit of a connoisseur of parodies! Over and above everything else I do feel strongly that a parody should respect (not just mirror) the original if it is to achieve any modicum of integrity (and literacy, by the way). In the end, whether it is "funny" or not will depend partly on one's knowledge of the original, certainly, but also - as with all humour or intended humour - it'll be a matter of personal taste. And not all parodies are actually intended to be funny - so obvious "laughs" shouldn't be forced into the material.
I firmly believe that good parody (as opposed to pastiche on one hand and cheap satire on the other) is (or can be) an art-form in itself. The main strand of parody writing already identified here on the thread - ie the knowing and generally astute paraphrase with relevant content that genuinely has something to say - may well include something from the "not the fields of Athenry" sub-set, and either observation, if well made, can succeed, and often on more than one level. Mostly though, the "topical rewrite" sub-genre is really less parody than cheap gibe, and many of these are blunt-egded or insubstantial and too transient in nature to be of much interest to anyone other than the folk historian in the long-term, and the political activist or the school playground (to which level they evidently aspire!)in the short-term.
Merely borrowing a tune and then fitting one's own views or statements doth not necessarily constitute parody, although there are examples which do so quite blatantly yet with due respect and knowledge (and sometimes also love) of the original, and thus encompass the best of both worlds.

As for the performance issue, well - first, the effective performance of parodies can be very much an art in itself too, and in a few instances the parodists themselves aren't necessarily the best performers of their own work (eg Les Barker singing? no way....) although in other instances they can provide insights that others will miss. Second, it's never easy to pick the right moment to sing a parody in a singaround. Only very rarely will it feel(or be) right to perform a parody after the original, and even then the singer must know his/her audience well for it to work as anything other than a lapse of taste. But I still maintain that a good parody at the right moment and/or with the right audience (not necessarily an in-crowd) is worth a dozen hand-picked and superbly sung serious ballads. Or can be.

Oh well, that's my two-pennorth for now.
MM

(Oh Leadfingers - you've got me hooked now, I must hear your Good Old Way!!! - can you PM me the words??)