Tenant has the place filthy

He's a licensee not a tenant if you also live there.

And yes, you can show him the door

Just make sure you change the locks when he's gone

He basically has no rights if you own the house and live there. Your rights as home owner trump his rights as a tenant. You only have to give him notice to have him gone.

https://www.citizensinformation.ie/en/housing/renting_a_home/sharing_accommodation_with_your_landlord.html#la4956

Hang on. All the advise above it based on it being your house.

But if your sister lives in the house it is the same advice. However if tenant is only in house since June and depending on contract or no contract advice may be the same even if sister is not in house

Then ignore all the advise and start again. You are not the landlord , your sister is.

And she's the one you need to report issues to

Not correct. The RTB made a ruling a few years back that you didn't need to be the home owner to have licensees. If the OP is "renting to" the "tenant" then the "tenant" is still actually a licensee

You're sister owns the house but isn't living there?

If that's the situation, you are a tenant/leaseholder and your problem licensee/tenant is actually a subletting tenant.

Has the filthy one been issued a rentbook? Do they know you're not the property owner? Is the tenancy registered as such with the RTB?

Have a read here, but long story short? If they are a tenant? And given you don't own the house? It would appear they are. You are in a more difficult situation to get rid, particularly if they are in situ over 6 months

The problem is that they can request tenant rights after 6 months even if subletting. Better to ask them to leave before it becomes a potential issue, especially if they are already going on about their rights.

Not if there's a lease which the OP hasn't clarified yet.

What's the whole story here?

If the house-mate is there since June, did they ever pay rent or is that a recent thing? Are they a partner / BF/ GF of another house-mate/tenant? There are so many variables.