Can this word be used to say something is mine.
Like possessive and a bit perhaps greedy?

I have been told by an Irish teacher that it can be used and more than just mine but more like "mine!"
"liomsa!"

?

Yes. "(Is) liomsa (é)" means "it is mine" the emphatic form of "Is liom é" (which doesn't sound right unemphasised).
"liom" is actually a "prepositional pronoun" the preposition being "le" and the pronoun "mé". So although "Is liomsa é" means "It is mine!" literally it only means "It is with me!". Le + mé = liom.

Is liomsa an carr (The car is mine.)
Ní liomsa an rothar (The bicyle is not mine)
An leatsa an bád? (Do you own the boat? / Is the boat yours?)
Nach leatsa é? (Is it not yours? -- expecting the answer "Ní liom" = "No!)
Cé leis é seo? (Who owns this?)

Irish, like English, employs prepositions, but with Irish, for each preposition (to, with, over, through,etc.) there is a form for 1st person (me), 2nd person (you), etc.

The basic preposition in this case is "le." For example one way to say "please" in Irish is "le do thoil," which means "with your pleasure."

The first-person form is liom, which means "with me." "Liomsa" is the emphatic form of "liom." So, liomsa, would mean "with me!"

Someone more knowledgeable can confirm whether "Liomsa" means "mine!" but I hope this gives you the basic idea.

Looks like someone more knowledgeable had already posted an answer while I was typing mine.

Thank you for the answers
so Liomsa will do, the other question was spelling
liomse that isn't a way to spell it either is it? Just Liomsa ?Yes Thanks all for your clarification and input.

"~~ se" and "~~sa" arise from the "caol le caol agus leathan le leathan" rule.

I think it would be better to say "'s liom(sa) é", or "'s liom(sa) sin!" but not "liomsa" alone.

The deabate about "Liomsa" reminds me of a story that a colleague of mine related to me.

His daughter, who was five years old at the time, came home from school one day and asked: "Dad, if you lose your lunchbox in the classroom, and you want to get it back, why do you have to tell teacher that it's lumps o' hay"

Mystified, Dad asked for more explanation!

"Well" said the daughter "If teacher finds a lunch box she holds it up and says 'Cé leis an lón seo?'. If it's your lunch you have to say 'It's lumps o' hay'

It's lumps o' hay : Is liomsa é

True story!