No legal practitioner qualified to practise in any court, whether within the Republic or elsewhere, shall be competent, without the consent of the person concerned, to give evidence at criminal proceedings against any person by whom he is professionally employed or consulted as to any fact, matter or thing with regard to which such practitioner would not on the thirtieth day of May, 1961, by reason of such employment or consultation, have been competent to give evidence without such consent: Provided that such legal practitioner shall be competent and compellable to give evidence as to any fact, matter or thing which relates to or is connected with the commission of any offence with which the person by whom such legal practitioner is professionally employed or consulted, is charged, if such fact, matter or thing came to the knowledge of such legal practitioner before he was professionally employed or consulted with reference to the defence of the person concerned.