I don't know what it is, but I have lots of them in my lawn, and I bin them. A fast-growing plant that you didn't plant and don't recognise is usually a weed.
Quote from above link......."Ragwort is a highly poisonous plant if eaten. Ragwort is toxic to cattle, horses, deer, goats, pigs and chickens. Sheep are less affected by it but some trials would suggest lower thrive due to eating ragwort.The poisonous substances in ragwort are toxic alkaloids (Jacobine, Jacodine and Jaconine). These cause the liver to accumulate copper, causing ill heath and death. On good pastures livestock avoid eating ragwort, as it is unpalatable, but where there is over-stocking and grass is scarce the weed is unavoidably eaten. The poisonous material contained in ragwort is not destroyed by drying. Hay containing ragwort is particularly dangerous. Grass silage containing ragwort is also a serious source of poisoning. Cases of poisoning occurring in late winter and spring often result from the feeding for some months previously of hay or silage cut from ragwort infested swards"
Definitely harmless in the garden... we had cinnabar moth caterpillars on ragwort on our garden wall last year. Delightful! We cut off the seed heads to stop it spreading onto neighbouring farmland.
Since 2019 I've lived in east Clare, in the west of Ireland.
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Hmmm!
In farmland lethal to horses when it is fresh or dried.
https://www.agriculture.gov.ie/media/migration/farmingsectors/crops/controlofnoxiousweeds/RagwortInformationSheet210317.pdf
Quote from above link......."Ragwort is a highly poisonous plant if eaten. Ragwort is toxic to cattle, horses, deer, goats, pigs and chickens. Sheep are less affected by it but some trials would suggest lower thrive due to eating ragwort.The poisonous substances in ragwort are toxic alkaloids (Jacobine, Jacodine and Jaconine). These cause the liver to accumulate copper, causing ill heath and death. On good pastures livestock avoid eating ragwort, as it is unpalatable, but where there is over-stocking and grass is scarce the weed is unavoidably eaten. The poisonous material contained in ragwort is not destroyed by drying. Hay containing ragwort is particularly dangerous. Grass silage containing ragwort is also a serious source of poisoning. Cases of poisoning occurring in late winter and spring often result from the feeding for some months previously of hay or silage cut from ragwort infested swards"