Denis Hennessy
Verified Expert in Engineering
Software Developer
Greystones, Ireland
Toptal member since November 4, 2020
Denis is a passionate engineer who loves building products that people actually use. In particular, he's been building iOS apps since the first iPhone launched. He has a high level of expertise in Swift and Objective-C. Since a modern iOS app is rarely a standalone entity, he's also worked with many external frameworks, including building REST endpoints where SDKs didn't already exist. He's also got experience building large server products, as well as some embedded-systems work.
Portfolio
Experience
- Linux - 20 years
- iOS - 12 years
- Objective-C - 10 years
- Swift - 6 years
- Python - 5 years
- REST - 5 years
- Cloud Firestore - 3 years
- SwiftUI - 2 years
Preferred Environment
Xcode, MacOS, Visual Studio Code (VS Code), Embedded C++, Raspberry Pi, Software, Full-stack, Mobile App Development, App Store Connect, APIs, React Native, RS-232, SDKs, Software Development Toolkits (SDK), Mobile, API Integration, Mobile Apps
The most amazing...
...app I've developed is an app that helps people suffering from bipolar disorder to understand their symptoms by recording their mood each day.
Work Experience
Mobile Systems Architect
Advance Local
- Managed the evolution of a suite of mobile consumer apps from a dated, Objective-C codebase to a modern Swift/SwiftUI platform (and Kotlin on Android).
- Acted both as a manager and architect for the mobile solution and as an individual contributor on the iOS codebase.
- Established a test suite and CI/CD pipeline, which enabled us to ship an average of over 500 releases per year.
Principal Developer
Peer Assembly
- Developed a medicine dose tracking app for a pharmaceutical company that provided consumers of the drug an easy-to-use way to track their usage while providing a significant market advantage for the company.
- Built a language-learning app for teaching Hebrew to children. I managed all phases of the project, from requirements capture and development to testing and successful launch.
- Developed a mobile game for a US fast-food chain. Besides being fun to play, the game was also an important part of their customer loyalty program.
Principal iOS Developer
Major US Media Company
- Converted a large portfolio of apps from an ad-supported revenue strategy to a subscription-based offering.
- Managed a major project evolution from a primarily Objective-C codebase to a primarily Swift codebase, while significantly enhancing the app feature set.
- Established a continuous-integration system to handle the quality assurance and automated release process.
Principal iOS Developer
Makalu Inc
- Took over development of their iOS location bookmarking app, updating it with the latest iOS platform features and adding new photo capabilities.
- Developed the "event app" for a major annual rock concert under demanding schedule constraints.
- Developed an educational app which encouraged children to write letters to elected officials.
Principal iOS Developer
Time Marketing
- Developed an innovative wine tasting and cataloging app for serious wine enthusiasts.
- Worked with the project owner and external design freelancer to come up with a unique look and feature set.
- Managed the evolution of the app from a paid-up-front model to a free-with-IAP model.
CTO and Co-founder
Valista Inc
- Handled the architecture of Valista's range of telco-grade payment and messaging products.
- Managed a distributed development team with centers in Ireland, Sri Lanka, and California.
- Set up a continuous integration system to improve the quality of software releases and increase the pace of updates.
Experience
Lines of Zen
https://linesofzen.comRego - Bookmark Your Places
https://regoapp.comGoal Streaks
http://www.goalstreaks.comFocus Time
https://focustimeapp.comSkills
Libraries/APIs
iOS Native Libraries, GraphQL API, React
Tools
Xcode, MQTT, TestFlight, Claude, Claude Code
Languages
Swift, Objective-C, C, Embedded C, Python, Ruby, GraphQL, Embedded C++, TypeScript, Kotlin, C++, Java
Frameworks
iOS SDK, SwiftUI, UIKit, WidgetKit, App Intents, Django, React Native, Tailwind CSS, Core Data
Paradigms
REST, Automated Testing, Mobile Development, Back-end Architecture, Mobile UI Design, Model View ViewModel (MVVM)
Platforms
MacOS, Linux, iOS, Firebase, RevenueCat, Visual Studio Code (VS Code), Raspberry Pi, Mobile, Amazon Web Services (AWS), Android
Storage
SQLite, PostgreSQL, Cloud Firestore
Other
Software Development, CI/CD Pipelines, Architecture, Payment Gateways, Software Architecture, Technical Leadership, Internet of Things (IoT), Web Development, App Store, Deep Linking, App Store Acceptance Process, iOS App Development, Native Mobile Apps, User Experience (UX), User Interface (UI), Bug Fixes, CTO, Fractional CTO, Large Language Models (LLMs), Agentic AI, Artificial Intelligence (AI), Software, SDK Development, AVFoundation, Full-stack, Video Processing, Video Streaming, Mobile App Development, App Store Connect, APIs, Full-stack Development, Embedded Software, RS-232, SDKs, Software Development Toolkits (SDK), IT Security, Scalability, Payment APIs, API Integration, Mobile Apps, Supabase, Embedded Systems, Firmware, Serial Peripheral Interface (SPI), Back-end Development, eCommerce, ESP32, FreeRTOS, Analytics, Publishing, Media
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