It might be worth asking your local postman if he has a name and address for the previous owners as you would like to contact them. If the litter is an ongoing situation, contact the relevant county council and outline your concerns. Litter left lying around constitutes a health risk as it will draw rodents and they can be forced to clean up. Check the electoral roll in the library and you may be able to get the previous owner's name from that and hence contact him. Also check with the council to know if they keep a register of rented properties, I'm not sure about that, if he is not with PRTB, chances are he is not registered.
If I were you , in the meantime I would not engage with these people. If you and your neighbours have concerns re drugs etc. make an appointment with the local superintendent and outline these concerns. If these people are dangerous, pass on your concerns . We had a similar situation and the local gardai told us to have nothing to do with them. The house was raided and they were evicted. The whole thing in Ireland with landlords, antisocial behaviour from tenants and their total disregard for neighbours rights is disgraceful but as I read today that 1 in 4 TD's are landlords why am I not surprised that nothing is done. You have my sympathy.
Also remember that if you get nowhere when you have found said landlord , you can always report him to the taxman. If he is paying his correct taxes, no harm done and if he isn't , well ,he is making illegal income at everyone else's expense and as he obviously doesn't give a fiddler's about you and your neighbours , why should you care if he has a bit of hassle?