"A leanas" isn't an adjective, but a relative clause meaning "which follows"; it means the same as "a leanann", but "a leanas" is more or less a set phrase. "A" is the relative particle (as in "an gasúr a itheann an t-arán"); "leanas" is the present relative form of the verb "lean" = "to follow". Any verb can replace the present tense ending "-nn" with "-s" when used in a direct relative clause, although this usage isn't obligatory in the Standard language ("an gasúr a itheas an t-arán", "an cat a bhíos ag an doras gach maidin"). There is a relative form in the future tense too: "Cad é a dhéanfas tú?", "an duine a bheas ann amárach". In some subdialects, the "-s" is added directly onto the present tense ending rather than replacing it ("a itheanns"), and in some areas you will find that a verb with the relative ending isn't lenited after the relative particle (as was also the case in Old Irish).