The simple answer is that, if you look up an adjective in the grammar section on teanglann.ie, you will see the comparative form (bigger, smaller, nicer, flatter, etc) listed with "níos" and the superlative form (biggest, smallest, nicest, flattest, etc) listed with "is".
The slightly more complex answer is that "níos" is a present tense form - in the past tense, it is "ní ba" ("bhí sé anseo ní ba luaithe" - "he was here earlier").
Gramadach na Gaeilge makes the case that strictly speaking, there is no superlative form, just different grades of comparative.
Yes, that phrase is tricky for some. It is the Irish equivalent of "Scheveningen" in Dutch. During the last war, inhabitants who were suspected of being German were asked to pronounce the name of this Dutch location. I have heard the efforts of some native German speakers as they fail to get their throats around it.
There are eight main declensions of adjectives, and the masculine vs. feminine distinctions are only in the genitive singular. The distinctions between the eight types aren’t always apparent from the nominative singular form, so it’s better just to learn it. (Note that all adjectives that end in a vowel except breá and te aren’t declined at all, and those two are only declined in the plural.)
When describing people ,"pretty" describes a persons physical appearance - "nice" and "pretty" are not synonyms when describing people, and an ordinary looking person can be "nice" without being "pretty".
"nice" and "pretty" can be synonyms when describing things or places - "a nice little cottage" or "a nice ring" can often be equally described as "a pretty little cottage" or "a pretty ring".
Deas means "nice", but when used to describe a place or a thing, you could probably get away with translating it as "pretty" at least on occasion. But it would be misleading to translate deas as "pretty" when describing a person.
No, deise doesn't provide emphasis in this exercise.
"Tá mo dheartháir go deas - "My brother is nice"
"Tá mo dheartháir níos deise - "My brother is nicer"
"Tá mo dheartháir is deise - "My brother is nicest"
deise is the comparative form of deas, user with níos to form the "-er" form of the adjective, and with is to form the "-est" form.