This is interesting. You do not pay CGT on encashments from unit funds. You pay tax under the 'gross roll-up' regime and cannot offset losses against CGT.
The average price of units is irrelavant, as far as I can see, as the procedures for calculating the tax due are provided in the Taxes Consolidation Act 1997 as amended by section 53 of the Finance Act 2000
http://www.irishstatutebook.ie/2000/en/act/pub/0003/sec0053.html#sec53.
And as you can see from this, partial encashments are taxed as (((Benefit –( Allowable Premia*Benefit)/Value of Fund before partial encashment)*Relevant tax rate). There does not appear to be any reference to unit prices. (Unless some other Finance Act subsequently amended the TCA in this regard.)
Each time you make a partial encashment the Act says the allowable premia are then reduced to calculate gains in future chargeable events. What the EBS appears to have done is after your partial encashment of 5,500, they calculated the reduction in allowable premia as (Benefit*Allowable Premia)/Value of Fund, i.e. (5500*30.482)/20,505), i.e. 6,661. This reduced your allowable premia to 23,790, i.e. (30,482-6,661). When you made your second partial encashment EBS appear to have calculated your reduction in allowable premia as ((10,000*23,790)/19,394), i.e. by 12,267 to 11,524. When you made your third partial encashment you had a taxable amount of (2,000-((2,000*11,524)/14,952), i.e. 459 on which tax of 188 is payable. So the reason you are being charged tax is because of the way the EBS are reducing your allowable premia.
Now I'm not a tax consultant and far be it from me to say the EBS are not following the Act but I suggest it would be prudent to ask them how they are calculating the reduction in allowable premia
where no gain has been made (i.e. as in your case) and if this is as provided for the the Taxes Consolidation Act 1997 as amended.