Yes, it’s ‘shóide’ because ‘deoch’ is a feminine noun. Compare for instance with ‘uisce sóide’ where there’s no lenition, because ‘uisce’ is a masculine noun. The plural of ‘deoch shóide’ would be ‘deochanna sóide’. ‘Sóide’ doesn’t change, as it means ‘what kind of drink’, regardless of whether you want to use the singular or the plural, i.e. ‘a drink of soda’ or ‘drinks of soda’. The same happens for example in ‘teach leanna’ (ale house) – ‘tithe leanna’ (ale houses). The qualifier or the second noun doesn’t take the plural. As for declining the word ‘sóid’, it belongs to the 2nd declension like ‘súil’, so you would have nom.sg. ‘an tsóid’, gen.sg ‘na sóide’ and nom.&gen;.pl. ‘na sóideanna’. Hope that makes it clearer! 🙂