Not really. They are both social gatherings, but they are different types of occasions. In very simple terms, a céilí is a party with dancing, and a cóisir is a party with cake. Traditionally, a céilí was an informal gathering, usually in the evening, which often ended up with dancing, but a cóisir was an event that you were invited to, like a wedding reception or a birthday celebration.
The terms have some overlap, but they are not interchangable.
An inexact distinction is that go dtí (and go) is generally used when the destination is important (like “to”), and chun (and chuig) is generally used when the direction is important (like “toward(s)”), but note that “to” will often be preferred over “toward(s)” in many English translations of chun, e.g. Lig chugat an scéal! (“Pay attention to the story!”). Either can be used when the destination/direction is a place name, e.g. either go dtí an Fhrainc or chun na Fraince could be used for “to France”. I means “in” rather than “to”.
I don't think it's a bug. People just need different amounts of repitition (e.g. I usually need only a few repititions to learn nouns refering to concrete, material objects that are easy to imagine, but I do need all the levels to remember verbs). We're all different :) You're free to move on after level 1.
Does anyone know how to find out the root of a verb? As far as I understand, there's only 1 root for each verb, but several stems.
The stems are shown on nualeargais.ie (e.g. present stem té-, preterite stem chuaigh).
But I don't know how to figure out the root - is it part of the verbal noun, since there's no infinitive? And how do I recognize the verbal noun in the first place? Is it the word shown as translation of the infinitive on focloir.ie (e.g. téigh or maybe gabh)?
There is some confusion of terms here. The "root" is the form that you will find in the dictionary. It is the second person singular imperative form of the verb. ól, rith, léigh, scríobh, tabhair, tiomáin, foghlaim, ceistigh, ceannaigh, oscail are examples of some of the verbs that you have encountered on Duolingo, and that you can look up in a dictionary. The "stem" is the bit of the root that is used when conjugating the verb in different tenses. (In fairness, some of the Tips & Notes on Duolingo aren't entirely consistent on this point).
For 1st conjugation verbs, the stem and the root are often the same (some exceptions). - ólann tú, ólfaidh mé - you all the present tense suffix ann and the past tense faidh onto the "stem". For 2nd conjugation verbs, it's a little bit more complex - oscail becomes osclaíonn in the present tense, and osclóidh - the stem is oscl.
The rules explain how to figure out the stem when you know the root. Figuring out the root if you only know the verb (and can therefor see the stem) is usually pretty obvious, though 2nd conjugation verbs can sometimes be a bit tricky.
Obviously for irregular verbs, the concept of "stem" doesn't work quite the same way - having different "stems" in different tenses is one of the things that makes them irregular. You can't "figure out" that téigh is the root of rachaidh, you have to learn that.
In English, the dictionary entry for a verb is usually the infinitive form of that verb. Irish uses the verbal noun where you would use the infinitive in English, and the verbal noun is not directly based on the stem or the root of a verb - they might be the same, they might not. Common verbal nouns often have an entry in the dictionary as well, as nouns. The verbal noun of léigh is léamh, the verbal noun of téigh is dul, the verbal noun of oscail is oscailt. You need to learn the verbal noun, though you usually "learn by doing", picking it up through usage.