This and a close relative Acaena novae-zelandiae are native to New Zealand, Australia and South America. They have both become naturalised in the wild in Northern Ireland and other parts of the UK. Often found in gardens and the picture here was taken at Mount Stewart a famous local National Trust property. They are low-growing evergreen, perennial subshrubs about 10 cm high which spread over the surface by rooting stems. They can become a nuisance on sand dunes where the burrs are painful to bare feet; and in forestry plantations. It is illegal to plant them in the wild.
The leaves are usually bright green, arranged alternately on the stems and are pinnate with a number of leaflets - usually about 11. There are a few varieties including one with a bronze tinge to the leaves.
In summer small flowers with no petals appear in spherical clusters on an upright stem above the foliage. They change from red to brown as they mature, and have spiny hooks which attach to animal fur as a means of transfer - it is believed that they may have come to this country in the wool of imported sheep.

The infestation is worst in soil with low fertility as Acaena is easily out-competed by fast-growing grasses. Cultivating the soil can spread fragments of stem which will take root.
A systemic weedkiller such as Glyphosate should eradicate it. In the garden it is most likely to be an escapee from a rock garden where it may have been plantad as ground-cover.