"Brendan" is English so you wouldn't normally lenite it or otherwise modify it.

The Irish equivalent is Breandán, which is changed as follows:

Breandán => a Bhreandáin
BiRHAN-dawn => uh VRHAN-dawñ
/b´r´æ:Ndɑ:N/ => /ə w´r´æ:Ndɑ:n´/

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

The stupid stuff you do when you type too fast!

Go raibh maith agat!

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Tuighe?

Yes because a Thom isn't clear in speech, names wont be lenited if it would sound awkward/difficult.

Well, if people were used to, it would be clear... people do say "a Sheáin", "a Shíle", "a Shéamais", "a Fhearghail", although they sound quite different from the unlenited forms.
But to me "Tom" is simply an English diminutive of Thomas, it's not an Irish one (which is "Tomáisín", I guess), and I think it's the reason it isn't lenited. The initial T is pronounced as in English, isn't it?

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

'Sé mo thuiscint féin nach gcuirtear rialacha gramadaí na Gaeilge i bhfeidhm ar fhocail Béarla. Ní hé sin le rá nach ndéantar é i rith cainte nádúrtha.

I used to go to a school in Cearnóg Pharnell in Dublin. That was always the given address. As far as I know, the road signs (which can be outrageously wrong, like most - yes most - signs in Irish!) now say Cearnóg Parnell. AFAIK this is correct. Foreign names should not be lenited in standard Irish. There is a logic to this, but equally there is a logic to people who speak Irish wanting to apply the normal rules of Irish to these words - adding a h to show possession just comes naturally. On the other hand the TV 'star', Hector, who is not a native speaker by any means was interviewing Bob Geldof and said 'Slán, a Bhob' (pronounced wob!) at the end. It sounded (purposely) ridiculous.

So that clears up nothing at all, but shure tis all about the discussion.

What an excellent ending to a contribution!

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So it sounds like a native speaker in an Irish speaking area would probably lenite "Brendan" even though it's not technically correct since it's an English spelling?

I'd certainly think so, especially given that it's pretty close to a Bhreandáin.