Personally I would say "madraí uisce". We do use the R in the plural. As for the hyphen, I don't know.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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That's interesting! So you have Mada, Madraí. That seems to happen a lot in Irish, the singular changes, but the plural stays the same.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

I'm thinking that madadh was the original word, and that the -raidhe ending (in the old spelling) is essentially the same collective plural ending in gasra, iníonra and similar words for a pack of dogs, and that in Munster the r has "infected" the singular?

Apparently both are in Middle Irish: Matad and Madrad. However Old Irish only had Matad (which became Mada), so Conamara has the original word and Madra developed sometime between 10th-12th centuries.

In Classical Irish Madra had Madrae as the genitive, which was pronounced exactly the same but took seo/sin instead of so/san.

The plural was also Madrae, but some writers trying to be old-fashioned wrote Madradha in an attempt to guess the Middle Irish form.

Only Madra and Giolla had this type of genitive (Madrae, Giollae) in Classical Irish. Also in Classical Irish you were allowed use Mada or Madra.

EDIT: References: Damian MacManus' essay in Stair na Gaeilge, 4.13. Also the Irish grammatical tracts, section 80.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal