It's to do with the fact that many ancient people had no word for "green". The Welsh for grass is glaswellt, gwellt being "straw", so grass is "blue straw". I believe it's true in many languages, not just Celtic and Brythionic.
See Wiki: https://en.wikipedia.org/wiki/Green#Languages_where_green_and_blue_are_one_color
It makes you wonder what was different about light or human eyes that there was no (or little) distinction between the colour of the sky and the colour of grass/trees.....
Has nothing to do with light hitting the retina or anything, I grew up speaking Vietnamese and the word for green is the same for blue. We can distiguish both colors, we just say "leaf green/blue" or "ocean green/blue" or when we get specific into shade we can describe it further.
As a linguist, though I don't exactly work as one and have forgotten a lot, I can say that it's not that they don't have a word for green, but the word for green develops before the word for blue. There's a Tom Scott video about the so-called "grue", which explains it far better than I tried to. But basically this word appears after a language develops words for black, white and red. Then comes "grue" which refers to both green and blue, and some time later they split into 2 different words. And for me, a native Russian speaker, it's funny how we have separate words for light blue/azure and blue. And they are both in the rainbow.
They're different, but best to choose a dialect and hear for yourself:
(ignore the níos in the second one; it doesn't have an entry for fearr alone as it considers it a form of maith).
I don't hear much difference in the Munster dialect.
Is fearr liom is "I prefer", is fuath liom is "I hate", is aoibhinn liom (as in "really, really like", rather than a romantic attachment), is maith liom is "I like", and there are quite a few more qualifiers used in the same way.
Note that the is is very much part of the phrase. "love", "hate", "prefer" are all being used as verbs in those English phrases, and the is is the verb part of is maith liom.
Maybe someone that knows Irish better can explain this less academically, but some of the vowels are only there so you know how to pronounce the consonants - the broad vowels (a, o, and u) give a broad (velar) consonant, and slender vowels (e and i) give a slender (palatal) consonant. (http://en.wikipedia.org/wiki/Irish_phonology) Because of this you almost always see the same kind of vowel on either side of the consonant - hence "glasraí" and not "glasrí," which would leave the s and r ambiguous. Long vowels are always pronounced, so in many cases that helps you identify which vowels take precedence.
In my (very limited!!) experience, vowels tend to run together in Irish pronunciation. "Aoi" is present in a lot of words - nearly always it is pronounced "ee", or at least around Clare/Limerick where I live. As far as I can think, "aí" is always an "ee" sound. But I'm an Aussie so I may most definitely have it wrong :) Spelling is pretty complicated in Irish, but certain vowel combinations always make the same sounds, so look for patterns!!
The copula (that is to say, the "is" in your sentence) is used to say that something is equal to another thing, or that something is a member of a group of other things - for example, I am a woman, that is our egg, it is an elephant, those are vegetables.
But even if we swapped out the is and replaced it with tá, we would get Tá glasraí liom. That translates as "Vegetables are with me", which makes no more sense in Irish than it does in English!