This isn’t really a joke as such, but it is a pun, centered around a not-often-seen meaning of the word Irish. The OED article on Irish has this in sense A.5.c (adj.):
colloq. (somewhat offensive). Of a statement or action: paradoxical; illogical or apparently so.
The speaker is presumably talking about boxing and thus a fist blow, but he measures its force in foot-energy. Therefore he adds “if I may say it like that without coming off too illogical, giving foot values to hand blows”.
As Compro01 points out in the comments, the foot-energy reference is most likely to the Imperial unit foot-pound, which measures energy (“the energy transferred on applying a force of one pound-force (lbf) through a displacement of one foot”), corresponding to the metric joule.
This actually makes the pun a double-pun, as it were: the unit of measurement relates to the foot as a unit of length, not the physical body part—but the speaker here puns on the dual meaning of the word foot to create a second pun on hand vs. foot.
There may even be a third layer to this pun, since Irish can also have overtones of quick-temperedness and violence; sense B.5 (n.) in the OED entry reads:
colloq. (orig. U.S.). Fieriness of temper; passion, anger, rage. Chiefly with up, esp. in to get one's Irish up.
Considering that the reference here is to a fist blow compared to feet (and thus implicitly to kicks), it may be that Irish here is intended not only to mean ‘paradoxical, illogical’, but also in a second, deeper layer ‘temperamental, violent’.