Consider the two statements
The first statement is a fact. You can find this in GAA records or even watch the highlights online. A fact is something that can be checked and backed up with evidence. Facts are things that are true and are accepted as such. Nobody will assert that Dublin didn't win the 2019 All-Ireland Senior Football Championship. You can perhaps say that they didn't deserve to win it, but that's a different story (and an example of an opinion).
The second statement is an opinion. An opinion is based on a belief or view that is not based on evidence that can be checked. There's no fact source stating that the statement is true. You might see it in writing, but it's still an opinion held by the writer. Other people might think that another player might be a better free taker. It's a good topic for conversation in a pub, perhaps.
If you did state that Dean Rock is the best free taker, you'd probably be asked to state why you have this opinion. Perhaps he has the best success percentage, or perhaps he's scored more frees than anyone else playing inter-county football right now. You can't just say "because he just is the best, right!"
A lot of the time in academic writing, you are going to be asked to state your opinion and justify it. To justify your opinion, you need to support it with facts. To get the facts you will need to do some research. In academic writing, your opinion supported by facts is called an argument.
A piece of academic writing tells a story. For a story to be a good story, as well as a beginning, a middle and an end, it needs to be convincing. You're probably writing in response to a question, so you have to answer the question and then justify your answer. You do this by building an argument: you state an opinion and use evidence to support that opinion. Your argument is how you express your point of view and answer the question you have been set, using evidence. Your argument should help you plan the structure of your writing and help find the evidence you need to support it. Any argument you make that does not have any evidence can and will be ignored.
Your argument should be the theme of your writing - everything that you include should be relevant to it. What you have to do is:
Here's an example, you want to argue that:
China will dominate the world's economy because of its abundant supply of rare earths (Wikipedia definition)
In support of this argument, you need to provide evidence. You then find sources that say:
These all support your argument, but you might find other sources that say
You then think critically about the sources that favour your argument and those that disagree with it and give good reasons why you agree or disagree with the sources. Including sources that disagree with your argument - counterarguments - actually strengthens your argument:
If you look online at other academic writing sites, you will see many mentions of a thesis statement. A thesis statement states the main argument of your work and describes, briefly, how you will prove your argument. Referring to the example above, China dominating the world's economy - that's your argument - and your approach to proving this is will be using its abundant supply of rare earths. A good thesis statement needs an argument (China dominating the world economy) and an approach (the importance of rare earths).
Here's another example. You might write that about the fall of the Roman Empire. A weak thesis statement might say the Roman Empire declined and fell because of a variety of interrelated factors. A stronger statement might say the Roman Empire declined and fell because of a succession of weakened Roman governments failing to deal with the rise of Germanic tribes encroaching on the Empire's borders.
A thesis statement makes a specific statement to the reader about what you will be trying to argue. Your thesis statement can be one or two sentences in length. You shouldn't state actual evidence or use examples in your thesis statement, simply include the argument and the approach.
A good thesis statement will help you with your academic writing by keeping you focussed on your argument and the approach you'll take.