This is something you may hear old people say in Ireland. It means: “He was there.”
Twas is obviously a contraction of It was. You will often here traditional Irish speakers preface their English declarative sentences with It, which has no function except to act as a statement marker, or a grammatical subject. Natural English speakers do a similar thing when they say It was raining. The It in that sentence doesn’t refer to anything, so it’s not a real pronoun. It’s just a word to stick at the start of the sentence so it feels right.
Himself is a common usage of Irish people speaking English; they often use reflexive pronouns not just for emhphasis, but to foreground phrases, or even just as grammatical subjects. This tendency comes directly from the Irish language, where this sort of thing happens all the time. You will often here Myself and Tommy went to town rather than Tommy and I went to town.
That Was is one of those Irish idiosyncrasies that is regularly mistaken for shiftiness or verbosity, but is in fact merely a reflection of how Irish grammar is organised. It seems circuitous and evasive to English speakers, because if they used it, it would be. Similar structures in other languages include the wonderful French Qu’est-ce que c’est? [KES-kuh SAY] which literally means What is that which it is? where an English person would have What is it? There is no implication of evasiveness (or even prolixity) on the part of the French speaker - it’s just how their grammar is organised. And so it is with Irish, even when they translate the grammar to English.
In It is a frequent traditional Irish usage for There, because in Irish they are the same word. The Irish word for there or in it is ann [OUN]. When you think about it, they mean more or less the same thing, so it shouldn’t be that big a deal, but if you use this construction, it flags you as Irish. Ann is the third person singular of one of those forainmneacha reamhfhoclacha that I spoke about before, and will again, so we can all look forward to that.
The central point of this post is that the weird way some Irish dialects use English (as their native language, don’t forget) is not “wrong” or “stupid”. Rather, it’s directly informed by the way Irish is nailed together, and an attempt to make Irish grammar fit into English usage.
Now you know.