Is maith liom...I like.
How do you say I liked, as in past tense.

Ba mhaith liom, which also means I would like.
You may use other expressions as well but it depends on what you're talking about...

Because of the confusion of meaning alluded to by Lughaidh it is customary to translate "I liked" as "Thaitin [sé] liom." Ba mhaith liom is generally taken to mean "I would like".

I liked that film. Thaitin an scannán sin liom.

or
Bhain mé sult as an scannán sin

You can also say "Bhí dúil agam ann".

"Bhain mé sult as..." means "I enjoyed" so it may not be used everytime you have "I liked in English", I think. That's why I said other translations are possible but they may depend on what you're talking about.

or bhain mé sult as ....

As Lughaidh said,"bhain mé sult as" would have a similar meaning to "bhain mé taitneamh as",meaning "I enjoyed it."

I'd say "ba mhaith liom é" would be what you are looking for.

Or maybe not.

For example: I'd like to see that film = Ba mhaith liom an scannán sin a fheiceáil.

If it is a film you are talking about,you should say "Thaitin an scannán liom".
I liked

In this case, if you say "Ba mhaith liom an scannán sin", it would be ok as well because "I would like that film" wouldn't make much sense.

You can also say "Bhí dúil agam ins an scannán sin."

"Dúil" in Munster, though, tends to have a slightly stronger meaning, a fondness, a liking, a slight craving for something. "Tá dúil sa bhraoinín aige": "He's quite fond of a drop (of drink)", "He drinks quite a bit" for example.