The situation would seem much the same as if the deceased had made a
gift of, say, 100 shares in X plc, requiring the executors to buy such
shares to satisfy the gift.
In those circumstances the beneficiary would be entitled to the cash
value of the shares so, in the present instance, the legatees may elect
to receive the sterling equivalent.
Whilst I agree with Andrew's thoughts, as the nature of such gifts has
been litigated in the past there may be case law setting out how the
legatee's cash equivalent is to be calculated.
Having said the above, as the will was made in Ireland it may be that
Irish Law will apply and not English Law. However, as the cases that
would inform the position are likely to have been decided in the 19th
century, the outcome might be the same whichever law applies.