If someone works tirelessly, they keep going for a long time without stopping and without seeming to get tired. The word is an adverb, so it goes with a verb: you work tirelessly, campaign tirelessly, fight tirelessly. Two things are packed inside it. One is time: this is not one long afternoon, it is weeks, years, sometimes a whole life. The other is feeling: the person wants to do it. Nobody is forcing them. That is why we say a doctor worked tirelessly to save people, and it sounds like praise. It is almost always praise, in fact. You will meet it in news stories, in speeches, in the words people say at a funeral, in a boss thanking a team. It is a slightly formal, warm word — you would write it in a card, but in ordinary talk with a friend you would more likely say "she never stops" or "he works so hard". One honest thing though: nobody is really tireless. The person does get tired. The word describes how it looks from outside — they kept going anyway. So it is a kind of gentle exaggeration that everyone accepts. Because it carries admiration, it sounds odd with a bad action; "he worked tirelessly to ruin her life" is possible, but the speaker is being ironic on purpose, and a listener will hear that twist.