I'm assuming your sister's UK pension does not arise from a UK public sector employment.
If so, under the Ireland/UK Double Taxation Agreement, if she is a resident of Ireland and not of the UK, anything she receives from her UK pension is taxable only in Ireland.
I think that as a matter of UK law only 25% of the pension could be taken as a tax-free lump sum, so UK income tax may be withheld from the other 75%. But, if so, I think your sister should be able to claim that back from the UK revenue, by pointing to the provisions of the UK/IRL DTA.
As a matter of Irish law, your sister can receive a maximum of €200,000 tax-free in retirement lump sums from all sources, including foreign sources. GBP 35,000 is about €41,500 so, assuming she hasn't already received other lump sums, she will be well within that limit. But the payment she receives from the UK will reduce the amount she can receive tax-free from any other pension arrangements that she may have.