law firm dealing with urd worth more than rs 5000 per day

No, a law firm, or an individual advocate providing legal services to a business entity is liable to GST under RCM . Moreover,if provided to other than a business entity(unregd person,or business turnover below 20 lacs) or to another law firm is exempted from levy of GST as per S No. 45 of Notification 12-2017 (CGST-Rate). On perusual of above two provisions, a law firm or an indiviual advocate is not liable to be registered under GST.

The liablility to register for a law firm or an individual advocate will arise only in case when it is providing services to a foreign based client .

Assuming you are talking about supplier being unregistered:----

If it is receiving services worth more than Rs 5000 a day, then aslo liability to register on law firms will not arise as supplier is unregistered, receiver that is law firm is unregistered, no question of GST.

But, i understand you are confused between supplies made to an unregistered person u/s 9(4) and the case where supplier itself is unregisterd. These two are complete different situations. The former while attracts RCM liability and provision of Rs 5000 per day, but the later is complete different situation and doesnot involve both RCM and provision of Rs 5000 per day. 

Its simple interpretation of law, you are in thought that GST should be levied in every transaction, either supplier or the receiver has to pay GST in every transaction. But, how can you bring the question of GST between 2 unregistered persons, where supplier and receiver both are unregistered.