With your surname, I would go with an uncomplicated relatively easy-to-say first name. [name]Imogen[/name] will probably complicate the pronunciation and spelling of the name for many people. [name]Imogen[/name] and McTiernan have the same number of syllables and end with the same sound, so it does not flow very well.
My first choices from your list would be:
[name]Leo[/name] McTiernan // [name]Leo[/name] [name]James[/name] McTiernan
[name]Clara[/name] McTiernan // [name]Clara[/name] [name]Eve[/name] McTiernan
My favorites from your list are [name]Leo[/name] and [name]Clara[/name]. Such cute names for a baby, and I think they’ll age well too! [name]Don[/name]'t have any middle suggestions at the moment, but I’m sure some of our fello berries will help. [name]Love[/name] your last name by the way!