You definitely need to contact the PRTB, it would have been a good idea when you served the 28 days notice on the tenant.
You cannot 'evict' him physically yourself either by changing the locks or ejecting him. Read some of the disputes on the PRTB website - this will give you some idea of the penalties involved should a landlord act illegally.
The Gardai will not get involved as it's a civil matter unless there is a risk of personal injury to you, i.e. the tenant attacks you when you arrive at the property.
Agree with NHG - go an talk to the tenant in a calm manner and find out first hand what the problem is. If you're afraid of him, take someone with you. Talk to the PRTB first to find out where you stand legally.
He has no squatters rights and if he's in trouble and can't pay the rent, perhaps he should seek assistance from Social Welfare.