I'm not familiar with the expression myself (not being of Murphy ancestry, so far as I know), but if you want the full translation, with the "in my heart part" as well, it would be:
Ó Murchú i mo chroí go deo.
or, with the older form of the name (before the spelling reforms in the mid-20th century):
Ó Murchadha i mo chroí go deo.
There is a fada (acute accent) over the "i" in chroí, which might be hard to spot (just like the fada's in the other words).
There were at least three separate Murphy "clans" (maybe more - it's the most numerous surname in Ireland). The "Ó" form was most common, but in Ulster, particularly in Armagh and Tyrone, the form was usually Mac Murchadha, so if you know that your ancestors came from there, you could use this instead;
Mac Murchadha i mo chroí go deo.
There's also an adjectival form of the name which could be used (with the Ó form or the Mac form), which is Murchadhach, giving you:
Murchadhach i mo chroí go deo.
It's like saying, "[I'm] one of the Murphy's", rather than "[I'm] an O'Murphy".
Almost everyone dropped the Ó or Mac in the 19th century, and Murphy is one of the surnames in Ireland which has been slowest to readopt the prefix(es), with most people who do so choosing the Ó form. Another anglicized form of the name was/is O'Morchoe, which is closer to the Irish pronunciation. It is believed that, in all cases, the name came from the Irish word murchadh, meaning "sea warrior", so Ó Murchadha/Ó Murchú would mean "descendant off the sea warrior".
