Is maith liom means "I like " literally "it is good with me" 'Is' is a verb called a copula , which is used instead of tá when Speaking about something in a permanent state, . Liom is a prepositional pronoun; preposition 'le' 'with' + me. Leat is ' le ' + you, etc. Lena is 'le' plus 'a'. personal pronoun for his , her or their. the same rules for possevive pronouns applies to Lena . That is : His cat- a chat, milk is good with his cat - Is maith lena chat bainne; milk is good with her cat, Is maith lena cat bainne or with their cat - is maith lena gcat bainne.
Check out galaxyrocker's explanation above. It is confusing when you get the question variants spaced out over a lesson but there is a way of telling by the grammar rules previously given for lenition and eclipses - chat (has undergone lenition due to being 'his'), cat (remains unchanged due to being 'hers'), and gcat (has undergone an eclipse due to being 'theirs'). Hope that helps rather than adding to the confusion.
Audio based questions on these variations would have been very useful and perhaps less cruel here for beginners like us, but we will come across it often enough in text only.