If I spend on a buff on Monastery of Hirana, and I have Warmog equipped, do I get the buff back?

No, you do not get the buff back.

When you spend a buff to trigger the ability on Monastery of Hirana, that buff is removed from your unit (Rule 702.2.b). Warmog's Armor does not have an ability that automatically replaces or restores a buff once it has been spent.

Here is the breakdown of how these interactions work:

1. Spending the Buff: When you spend a buff for the Monastery of Hirana's ability, the buff counter is removed from the unit (Rule 702.2.b).
2. Warmog's Armor Trigger: Warmog's Armor only grants a buff "When I conquer." It does not trigger when you spend a buff or when a buff is removed.
3. Future Conquers: If the unit with Warmog's Armor conquers again in the future, the equipment's ability will trigger again, and the unit will receive a new +1 Might buff (provided it does not already have one, as per Rule 702.3).

In summary, spending the buff removes it, and it will only be replaced if the unit performs another conquer action while equipped with Warmog's Armor.