Neither include their methods or sampling criteria in the reports themselves. However I read elsewhere that the SF report is based on taking a sample of similar houses in similar demographic areas and comparing the prices that they are sold for compared to the previous month.
I found a long paper written in 1999 by the ESRI which explains their methods although to be honest it's not very clear (aimed more at economists and mathematicians methinks)
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They make a mention of SF in the reoprt but it's only a single sentence:
"In Ireland, the Sherry FitzGerald Indicator of second-hand house prices is based on repeated assessment of a basket of 150 properties, originally sold through Sherry FitzGerald".
This seems to suggest that they just go out and make repeated valuations of the same houses and decide what they think it might go for at any particular time - this couldn't be true??