Rental income UK property

Hi
Looking for some advice and maybe best speaking to expert tax advisor? Not sure where to start. My sister has a house in London that she recently started to rent out (she lived it in herself before that) Rent is about 24k gbp PA and is totally mortgage free. All rent paid into her UK bank account. Revenue doesn't know about house or no rental income has been received into her irish account. What is her best approach pay UK tax but is she also liable for irish tax for any proceeds. She is hoping to buy retirement house in Spain in 5 years from rental income so all rental income will eventually be transferred to a Spanish bank account. Any advice or opinion on best approach?

Is she Irish resident and domiciled.

She will have to report the income to the Uk authorities as the property is located there. If Irish resident and domiciled she will have to report the income to the Irish Revenue and she will get a credit for the UK tax paid.

Yes she is irish resident and paying Irish PAYE. So she has to pay tax in both UK and Ireland?

Yip. She pays in UK and gets credit for this amount in Ireland. If the total Irish tax is €100 and she pays €40 in UK, then there is €60 due here.