Leighis also is a genitive singular, and means ‘of medicine, medicine’s’, and also is used as an adjective ‘pertaining to medicine’. The nominative is
leigheas.
So
leighis sí would mean ‘of medicine of a fairy mound’, and does not mean a lot on its own. As Irish does not like double genitives, and often does other things when it has sequences like ‘a son of a father of a brother of an uncle etc…’, I am not entirely sure if something like ‘effects of fairy medicine’ would be
éifeachtaí leighis sí or
éifeachtaí leigheas sí¹…
‘fairy medicine’ could be
leigheas sí, lit. ‘medicine of a fairy mound’ (although there might be better ways to say it, I am far from being native or even actually competent in the language).
‘healer fairy’ is
sióg leighis, lit. ‘fairy of medicine’.
¹
GnaG gives only examples with one of the nouns definite. Anyone care to comment what happens to ‘a X of a Y’? Can multiple genitive happen here?