Ireland Family Visit Visa Question on long duration

My sister in mid twenties is going to visit Ireland from India, and planning to stay for about 2 months. She is a employed and planning take leave for the duration.

I am stamp-4 holder and I am going to sponsor her. I am bit worried if we put accurate travel dates of 2 months in visa application, that might cause an rejection considering how she can get leaves for 2 months. Is that true or shouldn't be a problem ?

Or should we put travel dates of around 20 days instead as they grant 90 days visa in any case ? Any advise would be appreciated.

In case we put 20 days travel dates, and she gets 90 days visa, would immigration officer in Dublin check flight tickets to confirm if what she put in visa application matches with her return tickets ?

There is no right or wrong questions to visa applications. Visa officers assess these applications on a case-by-case basis. For someone who’s planning to stay for 2 months, you would want to prove that you have sufficient money to cover the stay; your employer knows about the leave (and confirms whether it is paid or unpaid leave); and prove you have family ties back in India so you won’t overstay.

In terms of the border control, you will have to tell the truth at that point. They may ask for accommodation bookings and return flights and they may call the sponsor to confirm the visit details, so lying just isn’t an option. They wouldn’t necessarily check the visa application but it is always an option. So be prepared to answer these questions if they come up.

Depending on the country which she applies from, the local Ireland embassy might have different leniency. It's better if she puts truthfully the date she's going to enter and leave. If she puts less than, the tourist visa granted might have earlier date she has to leave the country by. If she puts 20 days, the embassy might grant only for those 20 days or 22 days, then she can't stay longer than that.
I've always put more days of travel than less, because I've had experience where an embassy granted me strictly for the travel dates I put in application, of course granted there is the means of travel & complete documents that prove I have reason to return.

90 days visa is not guaranteed, it just means if you get 6 months visa, you can only stay 90 days max before you need to leave the country & reentry.

If you do get 90 days visa, usually immigration officer will not bother you with what you put in the application, they only care about the confirmed return ticket that doesn't exceed 90 days.