No. The plural form is different, it is làmhan for hands.
But as it’s explained in the tips and notes to the Numbers skill, the numeral dà:
- takes the singular form of the noun, and
- lenites the noun.
You can access the tips and notes on the web browser version of Duolingo at https://duolingo.com, and also on the https://duome.eu/tips/en/gd website. Unfortunately the tips might not be visible on the mobile app – so if you use Duolingo on a mobile device, you might want to open your web browser and head to duolingo.com for reading.
As for why dà takes the singular form – older Gaelic had a separate dual grammatical number that had to be used after dà. In many nouns the dual form was similar or identical to singular, so when it disappeared from the spoken language, it was replaced by singular, not by plural.
You can see the old dual form sometimes with feminine nouns in which it was identical to dative singular, so you’ll see aon bhròg for one shoe but often dà bhròig for two shoes (but sometimes you’ll also see – and especially hear – dà bhròg with singular nominative), and trì brògan with plural for three shoes; and similarly you may find aon làmh, dà làimh, trì làmhan.
I think dà broadens a narrow "l" (that's quite awkward) so the "l" in "leabhar" is narrow in "tha leabhar ann" and so is the "l" in "tha trì leabhraichean ann" but the "l" is broad in "tha dà leabhar ann". About 21 years ago some learners were almost driven crazy by things like that, and I wasn't really any help for them: for me there was no problem in pronouncing it, but writing it and explaining it so that people who hadn't learnt to say it could understand it was just about impossible. So I like Silmeth's description of it above.
There's a very useful set of information about grammar and spelling and so on available at http:// www.akerbeltz.org/index.phptitle=Beagan_gr%C3%A0mair It can give you a general idea of how the language hangs together. The author is of course the person who produced Am Faclair Beag (held at https://www.faclair.com/) and he has written other useful stuff on his website starting at http:// www.akerbeltz.org/index.php?title=Prìomh_dhuilleag
You can think of it as broadening, you’re right it kinda is… but it’s really also just lenition of slender L´.
Scottish Gaelic has two variants of slender (I believe that’s the common term) L sound, the fortis (unlenited) /L´/ which has the clearly palatal sound of [lʲ ~ ʎ] and lenis /l/ which is the alveolar [l] like in English.
So dà, which causes lenition, turns the palatal slender /L´/ into the ‘neutral’ /l/ – but it’s still different from the broad L of Gaelic which is [ɫ̪] – the dental velarized ‘dark L’.
You’ll also hear this ‘broadening’ effect in other contexts where you’d expect lenition: in past tense verbs, after possessives mo my, do your, a his, etc.