What about "suimiúil"? I was taught that "suimiúil" means "interesting". Are they synonymous?
'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
"Tá sé spéisiúil" can mean "He is interesting" or "It is interesting". However, "sé sin" specifically means "that", therefore; "Tá sé sin spéisiúil."
Sé means he/it. Tá means is/am (But not in all cases. Tá is for things that change, e.g. Tá tuirseach orm - I am tired. Is is for things that are permanent or less likely to change, e.g. Is capaill iad - they are horses)
Spéisiúil doesn't mean "special", it means "interesting." Speisialta is "special."
Do you always have to use the pronoun when you use "sin"? I was wondering since you don't do that in any other language that I know.
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.