Is English going to remain being an official language of the EU?
Interestingly enough, there is no official language in either England or the UK — although other languages do have official status in other parts of the UK.
I do hope the withdrawal of the UK will lead to respective citizens of Ireland and Malta enjoying and practicing their own language more, with English being promoted to the language for international cooperation, but nowhere really spoken at home in the EU, because, well, cosmic irony I guess.
That said, it might not longer be a "procedural" language (status currently only held by English, French and German). But even then, I wouldn't bet on it. English is too ingrained at this point.
Languages are such a political thing. The whole recent (century) history of Gaeilge is a reaction to UK influence.
My bet is yes, even if Ireland and Malta were leaving the EU: everybody knows English, only a few know any other language.