Hi Tararrr, The problem is that people tend to think that a word is a thing. They are so used to using the words in their own language that the idea that other languages might have other words or express things in other ways doesn't cross their mind. Resistance comes from Latin and originally meant 'holding back'. In English, it is used in biology, law enforcement or history. In Irish, there are various ways of expressing resistance, but most of the natural ways would use a phrase (in modern Irish, anyway - Old Irish was different in structure). For example, 'there was a lot of resistance to the new law' would be 'chuir an pobal in aghaidh an dlí nua'. There is a newly composed word which stands for resistance in the historical sense (such as The French Resistance), which is frithbheartaíocht. This is composed of a number of elements. Beart means an act or action, beartaíocht is an abstract noun from this (acting), and frith means against or anti, so it means to act against. But this is a dictionary word, not the kind of thing a native speaker would use. Mostly, a native speaker would talk about standing their ground against something - an fód a sheasamh. For your tattoo, you might choose something like SEAS AN FÓD, stand your ground.
However, if you're looking for an old Irish expression meaning 'resistance', there was this word, frithchathugud, which can be found here:
http://edil.qub.ac.uk/dictionary/results-new.php?srch=resistance&dictionary_choice=edil_2012
This might be your best bet if you're going to write it in ogham. If there's anything you want clarified, get back to us. As I'm sure you know, you don't want to make a mistake with something as permanent as a tattoo. As many people have done in the past! I put a link on another piece of advice for someone getting a tattoo here. I suggest you check that out, as it shows about seven badly botched Irish language tattoos! Slán go fóill!