I came across a "Mermaid's song" in a text:
ものみな眠るさ夜なかに
水底を離るることぞうれしけれ。
水のおもてを頭もて、
波立て遊ぶぞたのしけれ。
澄める大気をふるわせて、
互に高く呼びかわし
緑なす濡れ髪うちふるい、
乾かし遊ぶぞたのしけれ!
The bolded adjectives, うれし(い)、たのし(い)、both seem to be conjugated to izenkei. If that is so, can izenkei of shiku adjectives be used in exclamatory function? E.g. second line as "how happy (I) am to leave the depths!".
If it's not exclamatory, then what could it it mean here? And if it's not izenkei, then what else is it?