See below what S82 of the Succession Act 1965 says. This is the amended version. Offhand, it seems that the witness signature is valid as the witness is
not an intended beneficiary
nor is the witness's spouse or civil partner. Part VII of the same act covers the matter of wills - Sections 76 to 100. Link
http://www.irishstatutebook.ie/eli/1965/act/27/section/76/enacted/en/html
82.— (1) If a person attests the execution of a will, and any devise, bequest, estate, interest, gift, or appointment, of or affecting any property (other than charges and directions for the payment of any debt or debts) is given or made by the will to that person or his spouse F36 [ or civil partner ], that devise, bequest, estate, interest, gift, or appointment shall, so far only as concerns the person attesting the execution of the will, or the spouse F36 [ or civil partner ] of that person, or any person claiming under that person or spouse F36 [ or civil partner ], be utterly null and void.
As far as name duplication is concerned whoever drafted the will should have done it in such a way that any such ambiguity is eliminated. A date of birth or PPS number referring to the named entities would be simple devices to avoid ambiguity. Ambiguity in a will can cause trouble