Where was Saoirse's mother and why did she leave at the end in Song of the Sea?

Song of the Sea was quite amazing for children and parents alike, wonderfully drawn and animated. Very touching but oddly leaving a bad taste in the ending as it almost seemed cruel how the mother acted.

Mother Selkie (I don't remember her name) at the finally of the movie appears on the edge of the stone giant's shore after mysteriously disappearing in the sea for years. She pays special attention to Saoirse seeming to completely ignore Ben and Conor for what felt like like a relative eternity emotionally speaking, until finally giving them proper attention.

I simply didn't get where she was the entire time during the disappearance and why she could not stay. She was not of this world but what did that mean? Why did she start a family then with Conor knowing this? Seems cruel to do such a thing.

Very unsetting in such an otherwise beautiful movie.