How do I calculate the concentration of sulphuric acid by a titration experiment with sodium hydroxide?
Molarity of diluted $\ce {H2SO4}$ (solution 2): $\pu{0.0013798 mol}/\pu{0.025 L} = \pu{0.054172 M}$
  (I may be using the wrong volume, is it possible that I have to add the $\pu{25 ml}$ to the $\pu{23.81 ml}$ and divide by $\pu{0.04881 L}$?)


No, you used the correct volume since you want to know the concentration of the $\pu{25 mL}$ that you added to the flask.


  $\pu{0.54172 M}$ - Molarity of original solution of $\ce {H2SO4}$.


Correct.