No, you don't just add "-sa". Please do scroll slightly down in the link and review the "contrast form" in the first table:
This article also explains the use of "leis" (as in le an / le na) as well as "lena / lenar" and others, so it is quite relevant for this course.
As HazelOShea already explained, is liomsa an mála means "the bag is mine".
Is liomsa an mála - "The bag is mine/I own the bag"
is maith liom an mála - "I like the bag"
is aoibhinn liom an mála - "I love the bag"
(As "love" is a bit subjective in this case, some other adjectives besides aoibhinn can be used).
Your emphasis is wrong - you MIGHT be technically correct, because I haven't looked for a specific rule, and if I found one, I don't know if it would be a descriptive rule or a prescriptive rule.
But a quick check of the FGB and the EID provides examples without the contrastive form, so I guess it can be optional, but I haven't encountered it being used that way in real life.