'Suimiúil' does mean 'interesting' as well. Ó Dónaill gives 'neat and clean; comely, attractive' as the primary meaning of 'spéisiúil', and 'interesting as its second meaning. http://breis.focloir.ie/en/fgb/sp%C3%A9isi%C3%BAil
When "that" (or "those" or ) is being used as a pronoun, you usually use it with an actual pronoun in Irish.
Sometimes the pronoun comes before sin, and when "that" is the object of a verb, or you are using the copula, you use the object form of the pronoun.