As I shouldn't continue OT posting on another thread but was asked to explain the diferrence.
Here is a basic explanation of the differences between fixed term and part 4 leases, as I understand it. Note, I am not a lawyer.
If you do not have a fixed term lease then you automatically have a part 4 lease (from Part 4 of the Residential Tenancies Act 2004.)
This provides security of tenure. After 6 months in-situ without being asked to vacate by the landlord, you are then entitled to remain for up to 4 years.
Part 4 tenancies do allow limited specific reasons that a landlord may terminate a tenancy earlier than this. These include wanting to sell the property, or wanting to move themselves, or their family in.
Even with these exceptions the land-lord has to give you increasing notice depending on how long you have been there. From only 4 weeks if you have been there less than 6 months, up to 16 weeks if you have been there for more than 4 years.
These reasons for early termination apply ONLY to a part4 tenancy.
If you have a fixed term lease (ie you signed a lease for 6 months, a year, whatever, instead of defaulting to a Part 4) then the above reasons for allowing the early termination of a Part 4 tenancy do not apply to you as you don't have a Part 4 tenancy. EG if they were to sell the property, you would be allowed remain, and the terms of your lease would apply to your new landlord. Reference:http://www.threshold.ie/page.asp?menu=70&page=241
if a fixed term tenancy exists it cannot under any circumstances be terminated before the expiry of the term, unless the landlord or the tenant is in serious breach of the agreement.
Note: Fixed term leases may specifiy similar exceptions allowing early termination. Check your lease and decide if you want to accept them before you sign.
Additionally, at the end of a fixed term lease you are automatically entitled to a Part 4 tenancy. I have seen a fixed term lease with an added clause that stated that no such Part 4 tenancy could arrise at the end of the lease but I have no idea if this is a right that cannot be given up. So I cannot say as to whether that is valid or not.
You are supposed to give 1 to 3 months notice to your Landlord as to your intention to take up a Part 4 tenancy after your fixed term lease expires but even if you do not you are still entitled to the Part 4 tenancy. You're just rather inconsiderate and may be liable for expenses incurred by your landlord due to this. So give the correct notice.
Additionally here is a citizensinformation explanation of it.
Very much open to correction on this, as once again, I am not a lawyer. The main thing to remember is that Part 4 and fixed term leases are not the same beast, and that part 4 specific exceptions do not apply to fixed term leases.