The "ə" is a Mid-central vowel, which does sound similar to a "u" (close back rounded vowel), but from a linguistic standpoint they are considered different. I am sure it depends where in Ireland you are getting the pronunciation from too. "ə": https://en.wikipedia.org/wiki/Mid-central_vowel "u": https://en.wikipedia.org/wiki/Close_back_rounded_vowel IPA Vowel Table: https://en.wikipedia.org/wiki/Template:IPA_chart/table_vowels
No, go raibh maith agat doesn't mean "it was nice of you".
Go raibh is the subjunctive form of the verb bí. Wikipedia suggests that it means "may there be good at-you", but, because bí and ag are used to say "have" in Irish, "may you have good(ness)" is probably a better so-called literal translation