The India side: the whole gain at 12.5%
India taxes the full capital gain from your original cost. For long-term property sold on or after 23 July 2024, the rate is 12.5% without indexation, and as an NRI you get neither the resident option of 20% with indexation nor any currency relief for the rupee's fall, so the whole rupee gain is taxed. The buyer must deduct TDS under Section 195 on the gain, plus surcharge and cess, not the small 1% that applies to a resident seller.
Because the TDS is heavy, a lower-deduction certificate is worth getting so the buyer withholds closer to the real tax, and any excess is reclaimed by filing an Indian return. India taxes the gain whatever Ireland does; the Irish position then depends on your domicile and what you do with the money.
Ireland: 33%, but only if domiciled or remitted
Ireland's capital-gains tax is 33%, well above India's 12.5%, so the domicile question matters a great deal here. If you are a non-domiciled Irish resident, the Indian gain is taxed in Ireland only to the extent you remit the proceeds. So if you keep the sale proceeds in India, for example in your NRO account, the gain is outside the Irish charge entirely, and India's 12.5% is the only tax you pay. Remittance, bringing the money into Ireland, is the trigger, so the timing and the decision to remit are the planning levers.
If you are Irish-domiciled, Ireland taxes the gain as it arises at 33%, wherever the money is kept, and gives a credit for the India tax under the treaty. One subtlety: Ireland computes the gain in euro, converting the purchase and sale at their own dates, so movement in the euro-rupee rate can make the Irish gain differ from the Indian one, and the credit is capped at the Irish tax on the Irish-measured gain. The practical effect is that a domiciled resident pays close to 33% overall, with India's 12.5% credited against it.