fluent in English in a mater of months That's completely impossible. Those Spanish students you're thinking of are going to private English lessons, meaning either they themselves are choosing to or their parents are sending them there. Either way there is significant motivation and support behind them.

Also, people who learn English as a foreign language are self-motivated to do so, therefore they study, use it etc. Whereas only a handful of children in a Gaeilge classroom actually make an effort to learn the language. No matter what a teacher does, you will only learn something if you make an active effort. As they say, you can bring a horse to water but you can't make them drink.