Problem with small leaf weed

I have a small weed which is spreading rapidly through my grass lawn and is effectively taking over. Grass is not able to survive so now I just have ad odd blade of grass growing through it. It has a white/pinkish thin stem with small round green leaves. I have heard it called the “Glasnevin” weed, probably because it is growing in many gardens in the area. One representative in a Garden Centre obviously knew of the weeds existence and said it could be effectively treated by Winter Tar Oil or Winter Tar Wash BUT this product was NO longer available.