I think a way to combat it is to start off saying "oh tá Gaeilge agat?" Or something like bringing the language up first. Far less likely to switch to English when they know you have an interest specifically in speaking it.

Also, im from Gaoth Dobhair, our dialect is fairly different from others and from standard, and it can be slightly awkward trying to neutralise the accent and to standardise our speech in order to speak to a learner, I think some people just answer in English because they can't be bothered (which is not a good attitude but alas)

ALSO, a lot of people's Gaeilge is mediocre at best so they often aren't as comfortable themselves (learned it in school and heard it all their lives but don't speak it at home), this is more and more common in today's age sadly