Because the Irish realized that in order to be able to compete economically and politically, it was necessary to shed the Irish language as it was hampering Irish people from entering both business and politics.

Again, the loss of the Irish language was collateral damage on the way to a free state. It was a necessary step in order to be able to tackle the foreign rulers of the country.

Even if the above is true why was it necessary to 'shed' Irish in order to make use of English?

Is it impossible for a person to be a competent speaker of both languages?

I think it's obvious that the majority of Irish people who abandoned their language didn't do so as part of a plan to become prosperous business people or to enter politics but as a result of the message that everything Irish - language, culture, traditions, music, pastimes etc - was inferior, worthless, backward, uncivilised, primitive etc being very successfully driven home to them.